UNIVERSITY  OF  CALIFORNIA 
LOS  ANGELES 


Gift 


A.  Guayule  field  with  all  plants  above  40  cm.  removed.     Lomas  of  Sierra  Zuluaga. 

B.  Guayule  field  of  maximum  density,  near  Apizolaya. 


GUAYULE 

(PARTHENIUM  ARGENTATUM  GRAY) 

A  RUBBER-PLANT  OF  THE  CHIHUAHUAN  DESERT 


BY 

FRANCIS   ERNEST   LLOYD 

Professor  of  Plant  Physiology,  Alabama  Polytechnic  Institute 


WASHINGTON,  D.  C. 

PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 
191 1 


CARNEGIE  INSTITUTION  OF  WASHINGTON 

PUBLICATION  No.  139 


Copies  of  this  Book 
were  first  issued 

JUL27I911 


PRESS  OF  J.  B.  LIPPINCOTT  COMPANY 
PHILADELPHIA,  PA. 


SB 
a/?  I 


PREFACE. 


In  1907  the  author  of  the  present  paper  was  engaged  by  the 
Continental- Mexican  Rubber  Company  and  the  Intercontinental  Rub- 
ber Company  to  organize  investigations  looking  toward  the  successful 
cultivation  of  a  Mexican  desert  rubber  plant,  the  guayule  (Parthenium 
argentatum  Gray).  Dr.  Theodore  Whittelsey  and  Dr.  J.  E.  Kirkwood 
later  became  identified  with  this  undertaking,  the  former  as  chemist, 
the  latter  as  assistant  botanist.  The  headquarters  for  the  investiga- 
tions were  established  at  the  Hacienda  de  Cedros,  Partido  de  Mazapil, 
Zacatecas,  Mexico.  It  was  not  a  matter  for  congratulation  that,  at  the 
close  of  a  year,  the  directors  found  it  inadvisable,  for  financial  reasons 
consequent  on  the  panic  of  1907,  to  continue  the  department  of  inves- 
tigation. By  the  courtesy  of  the  company,  however,  the  author  carried 
on  his  studies  for  some  three  months  beyond  the  termination  of  his 
business  relations  with  it,  and  this  period,  falling  during  the  growing 
season  of  1908,  brought  to  light  many  important  facts.  Still  further 
observations  of  capital  importance,  in  part  on  experiments  begun  in 
1907  and  1908,  were  made  by  the  writer  in  April  1909,  while  represent- 
ing the  United  States  Rubber  Company,  a  commission  which  could  not 
have  been  prosecuted  without  the  kind  concurrence  of  President  C.  C. 
Thach  and  a  number  of  the  writer's  colleagues  at  the  Alabama  Polytech- 
nic Institute.  As  silence  was  not  imposed  by  the  United  States  Rubber 
Company,  it  has  been  possible  to  include  these  observations. 

No  less  than  hearty  recognition  is  due  also  to  Mr.  W.  H.  Stay  ton, 
formerly  captain,  U.  S.  Navy,  sometime  president  of  the  Continental- 
Mexican  Rubber  Company,  and  now  president  of  the  Texas  Rubber 
Company.  It  is  stating  an  open  secret  to  say  that  it  was  through  the 
initiative  and  enthusiasm  of  this  gentleman  that  the  work  of  the  inves- 
tigation was  undertaken  and  would  have  been  continued  but  for  cir- 
cumstances beyond  his  control.  Mr.  Stayton  has  shown  a  liberal  and 
scientific  spirit,  qualities  not  of  necessity  nor  at  all  times  associated. 

Thanks  are  due  further  to  Prof.  J.  C.  Arthur  and  Prof.  W.  G.  Far- 
low  for  reports  on  various  pathological  matters;  to  Dr.  M.  T.  Cook  for 
contributing  manuscript  on  the  galls  found  on  guayule;  to  Dr.  A.  D. 
Hopkins  for  a  report  on  the  guayule  bark-borer;  to  Dr.  L.  O.  Howard 
and  Dr.  J.  G.  Sanders  for  the  identification  of  certain  insects;  and  to  Prof. 
B.  L.  Robinson  for  his  courtesy  in  causing  a  photograph  of  the  type  speci- 
men of  guayule  to  be  made.  Mr.  Charles  S.  Ridgway  has  rendered  sub- 
stantial aid  in  the  preparation  of  certain  figures. 

The  drawing  for  figure  5  was  supplied  by  Professor  Arthur;  the  nega- 
tive of  plate  2,  fig.  B,  was  made  by  Dr.  W.  E.  Hinds;  Professor  Trelease 
furnished  the  illustration  (fig.  4)  and  description  of  the  Cedros  sotol,  and 
kindly  made  several  other  determinations;  the  negatives  of  plate  3  and 

iii 


5O5086 


iv  Preface. 

plate  4,  fig.  A,  were  made  by  Mr.  Victor  Blanco.     Dr.  H.  van  der  Linde 
obtained  for  me  valuable  material  of  irrigated  plants  from  Caopas. 

Dr.  Theo.  Holm  has  afforded  me  the  benefit  of  his  criticism  of  the 
portion  of  this  work  treating  of  the  anatomy,  and  has  been  good  enough 
to  examine  inaccessible  literature  for  me.  Dr.  W.  E.  Safford  did  a  like 
service  regarding  a  few  pages  in  the  first  chapter. 

To  Prof.  W.  L.  Bray  I  am  indebted  for  information  about  the  Texas 
guayule  fields,  later  verified  by  me  personally ;  and  to  my  colleagues,  Prof. 
C.  L.  Hare  and  Prof.  J.  P.  C.  Southall,  for  assistance  in  making  chemical 
analyses  and  for  mathematical  formulae,  respectively. 

With  reference  to  the  chapters  which  follow,  no  pretensions  are 
made  with  regard  to  completeness.  The  exhaustive  study  of  a  single 
plant  from  all  points  of  view  might  well  be  numbered  among  the  labors 
of  fable.  The  reader  is  asked  also  to  remember  that  the  study  of  but 
a  single  growing-period  was  possible.  Much  of  the  experimentation, 
therefore,  was  done,  as  it  eventually  turned  out,  during  the  most  un- 
favorable season;  but  in  the  case  of  field  experiments  this  was  not 
entirely  a  misfortune.  That  the  theoretical  bearing  of  many  observa- 
tions and  more  refined  methods  of  making  them  are  less  attended  to  than 
the  matter  warrants  has  been  due  to  the  urgent  necessity  of  practical 
success.  With  these  qualifications,  the  work  may  be  regarded  as  a 
report  on  a  unique  opportunity,  unhappily  shortly  terminated,  to  bring 
a  hitherto  feral  desert  plant  under  the  subjugation  of  culture.  That  suc- 
cess may  ultimately  be  attained  is  not  an  unreasonable  nor  an  unwar- 
ranted expectation,  for  which  statement  the  interested  reader  will  find 
not  a  little  evidence  in  what  follows. 

FRANCIS  ERNEST  LLOYD. 
ALABAMA  POLYTECHNIC  INSTITUTE, 
January  1910. 


TABLE  OF  CONTENTS. 


Preface ',  {{\ 

List  of  plates vii 

CHAPTER  I. — HISTORICAL  ACCOUNT. 

Original  discovery  and  description 3 

The  vulgar  name 4 

Primitive  and  later  uses 5 

History  of  manufacture 7 

Methods  of  extraction     8 

The  natural  supply  of  shrub 10 

Attempts  at  culture 12 

CHAPTER  II. — THE  ENVIRONMENT. 

Geographical  distribution 13 

Altitudinal  distribution 13 

Climate 14 

Rainfall 14 

Air-temperatures 15 

Soil-temperatures 18 

Soil-moisture 19 

Relation  of  rainfall  and  temperature  to  growth 20 

Relative  humidity 21 

Topography  and  soil 23 

Density  of  growth 25 

Biotic  relations 35 

CHAPTER  III. — DESCRIPTION  OF  THE  GUAYULE. 

Seed 46 

Seedling 48 

The  mature  plant 50 

Root-system 50 

Retonos 51 

Method  of  branching 54 

Biotypes 55 

Size 56 

Surface  characters  of  the  stem  and  method  of  determining  age 57 

Field  plants 57 

Irrigated  plants 58 

The  leaves 58 

The  inflorescence  and  the  flowering  period 59 

The  production  of  seed 60 

CHAPTER  IV. — REPRODUCTION. 

Methods  of  reproduction 61 

Retonos,  normal  and  induced 61 

Seed 68 

Rate  of  reproduction  and  of  growth 75 

Rate  of  growth  during  germination 75 

Rate  of  growth  in  maturer  plants  beyond  the  seedling  stage 79 

Rate  of  growth  in  terms  of  stem-length 79 

Rate  of  growth  in  earlier  years  after  germination 79 

Rate  of  growth  in  medium-sized  plants 81 

Rate  of  growth  in  irrigated  plants 84 

Field  plants 85 

CHAPTER  V. — ANATOMY  AND  HISTOLOGY. 

Root 90 

Primary  structure 90 

Secondary  structure 90 

Hypocotyl 96 

Primary  structure 96 

Secondary  structure 99 

Later  secondary  structure 101 


vi  Table  of  Contents. 

CHAPTER  V. — ANATOMY  AND  HISTOLOGY — Continued. 

Page. 

Age  and  structure  in  the  seedling 104 

Epicotyl 105 

The  definitive  stem 107 

Primary  structure 107 

Secondary  structure 109 

Origin  of  the  medullary  and  cortical  stereome no 

Annular  structure 114 

The  effect  of  abundant  water  upon  anatomical  structure 116 

Relative  volumes  of  cortex  and  wood 117 

Effect  of  various  amounts  of  water  of  irrigation 121 

Effect  of  drought  following  irrigation 122 

Effect  of  irrigation  on  the  physical  characters  of  the  wood 122 

The  peduncle 124 

The  leaf 125 

Cotyledons 125 

Prophylls 126 

The  definitive  leaf 126 

CHAPTER  VI. — THE  RESIN-CANALS  IN  THE  GUAYULE. 

The  canal-systems 165 

Primary  canals  in  the  root  and  hypocotyl 165 

Primary  cortical  canals 166 

Medullary  canals 1 69 

The  canals  in  the  leaf 171 

Primary  canals  in  branches 171 

Secondary  canals  in  root,  hypocotyl,  and  stem 172 

Canals  in  the  peduncle 172 

The  canals  in  retonos 173 

The  contents  of  the  canals;  their  origin 174 

The  role  of  resin 174 

Resin-content  of  guayule  by  analysis 175 

CHAPTER  VII. — THE  ORIGIN  AND  OCCURRENCE  OF  RUBBER. 

Methods 176 

General  distribution  of  rubber  in  the  plant 177 

The  appearance  of  rubber  in  richly  loaded  tissues 179 

Behavior  of  peridermal  divisions  toward  rubber 179 

The  development  of  rubber  in  the  cell 180 

Centers  of  secretion 1 8 1 

Rate  of  rubber  secretion  relative  to  growth 183 

Rubber-content  by  chemical  methods 185 

Variation  in  relative  amount  of  rubber  in  field  plants 187 

Relation  of  rubber  and  resin 188 

The  significance  of  rubber 188 

Summary 190 

CHAPTER  VIII. — VEGETATIVE  REPRODUCTION. 

Induced  root-regeneration 193 

Propagation  by  cuttings 195 

CHAPTER  IX. — THE  CULTIVATION  OF  GUAYULE. 

Forestal  operations 199 

Present  field  operations 199 

Suggested  rules  of  practice 200 

Harvesting  period 202 

Reseeding  barren  ground 202 

Cultural  operations 203 

Seed \ 203 

The  raising  of  seedlings 203 

Irrigation 208 

Transplanting 209 

Harvesting  cultivated  guayule 210 

Catch  crops 210 

Bibliography 2II 


LIST   OF   PLATES. 


Facing 
page. 
PLATE  i.  A.  Guayule  field  with  all  plants  above  40  cm.  removed,  "j 

Lomas  of  Sierra  Zuluaga >  Frontispiece 

B.  Guayule  field  of  maximum  density,  near  Apizolaya  J 

2.  A.  The  type  specimen  of  Parthenium  argentatum  Gray \ 

B.  Transverse  section  of  a  very  old  stem f 

3.  A.  Upper  floor  in  a  guayule  factory  from  which  pebble-mills  are 

charged 

B.  Lower  floor:  discharge  chutes  and  ditch  from  pebble-mills  ..... 

C.  A  battery  of  washing  and  sheeting  machines j 

4.  A.   Stacks  of  guayule  in  bales.    Continental-Mexican  Rubber  Com-  ] 

pany 's  factory j        „ 

B.  Experimental  ground,  with  plants  two  years  old  from  stocks.  \ 
Cedros J 

5.  A.  Station  2,  Quadrats  5  and  6.     Lomas  of  Sierra  Zuluaga ) 

B.  Station  3,  Quadrat    i,  near  Cedros.     A  good  stand  of  mature  \      24 
plants J 

6.  Plants  from  Quadrats  5  and  6,  Station  2 24 

7.  A.  Quadrats  (Station  12)  with  very  dense  growth.     Apizolaya. . .  \ 

B .  The  same,  the  guayule  removed J      3 

8.  A.  Narrow  type  of  guayule \        , 

B.  Spreading  type  of  guayule /     •> 

9.  A.  The  root-system  of  guayule | 

B.  Groups  of  plants  which  started  as  retonos >•      48 

C.  A  strongly  monopodial  retono ) 

10.  An  exceptionally  tall  (130  cm.)  individual.     Caopas 52 

11.  A.  A  widely  spreading  (130  cm.)  plant  of  guayule ) 

B.  A  large  plant  of  the  usual  habit.     Apizolaya / 

12.  A  biotype  of  guayule.     The  winter  condition  on  the  left 52 

13.  A-C.  Seedlings  of  typical  and  atypical  guayule \        6 

D.  Seedlings  and  mature  plants  of  these  biotypes J 

14.  A.  An  irrigated  plant,  from  a  small  stock,  at  the  height  of  flowering  \        , 
B.  "Hembra"  (on  the  left)  and  "macho"  (on  the  right)  guayule.  J 

15.  A.  Induced  retonos  on  a  tap-root.     One  season's  growth \      , 

B.  Induced  retonos  on  a  lateral  root.    One  season's  growth / 

16.  A-C.  New  growths  after  pollarding \      fi8 

D.  Seedlings  in  limestone  soil;  E,  in  "garden"  soil / 

17.  A.  Minimum,  average,  and  maximum  seedlings.    (Station  2,  quad-) 

rat  4) [      68 

B.  Irrigated  plant,  two  years  old  from  a  stock.    Cedros J 

1 8.  Seedlings  growing  in  different  soils 72 

19.  Seedlings  growing  in  different  soils 72 

20.  A.   (i)  Root-cutting;  (2-4)  sectorial  root-stem  cuttings  (Exp.  146).  1 
B.  Seedlings  grown  in  different  soils / 

21.  A.  A  branch,  one  year's  growth  under  irrigation )      ,, 

B.  A  branch  in  the  height  of  flowering,  second  season J        4 


22.  Anatomical  and  histological  details,  figs. 

23.  Anatomical  and  histological  details,  figs. 

24.  Anatomical  and  histological  details,  figs. 

25.  Anatomical  and  histological  details,  figs. 

26.  Anatomical  and  histological  details,  figs. 

27.  Anatomical  and  histological  details,  figs. 

28.  Anatomical  and  histological  details,  figs. 

29.  Anatomical  and  histological  details,  figs. 

30.  Anatomical  and  histological  details,  figs. 

3 1 .  Anatomical  and  histological  details,  figs. 


to  16 128 

to  9 130 

to  13 132 

to  10 134 

to  13 136 

to  10 138 

to  5 140 

to  6 142 

to  ii 144 

to  14 146 

vii 


Vlll 


List  of  Plates. 


Facing 

page. 

PLATE  32.  Anatomical  and  histological  details,  figs,      to  7 148 

33.  Anatomical  and  histological  details,  figs,      to  10 150 

34.  Anatomical  and  histological  details,  figs,      to  9 152 

35.  Anatomical  and  histological  details,  figs,      to  15 154 

36.  Anatomical  and  histological  details,  figs,      to  8 1 56 

37.  Anatomical  and  histological  details,  figs,      to  8 158 

38.  Anatomical  and  histological  details,  figs,  i  to  18 160 

39.  Anatomical  and  histological  details,  figs,      to  8 162 

40.  i.  Rubber  in  canal-cells,  nearby  cortex  and  inner  ray-cells 

2 .  Older  root.     More  rubber  in  rays 

3 .  Root  2  mm.  diameter 

4.  Parenchyma  ray  from  fig.  2 

5.  Upper  part  of  hypocotyl,  same  age  as  fig.  i 

6.  Longitudinal  section  through  old  wood [-    172 

7.  Longitudinal  section  through  mature  leptome  parenchyma,  with 

a  few  parenchyma-ray  cells 

8.  Leptome;  elongated  elements 

9.  Companion  cells  and  sieve-tubes.    No  rubber  in  younger  leptome 

on  the  left 

41.  i,  2.  Cortex,  stem  of  field  plant  with  maximum  rubber-content. . 

3 .  Cortex  of  a  ao-year-old  stem 

4.  Root ;  rapidly  grown  seedling,  2  months  old.    Rubber  in  granules    *•    172 

5 .  Rubber  in  process  of  accumulation  in  an  irrigated  plant 

6.  Primary  resin-canal J 

42.  i.  Apex  of  terminal  twig  of  1908,  field  plant 

2.  Near  base  of  same 

3 .  Pseudotylosis  with  rubber  in  the  cells 

4.  Leptome,  field  plant 

5.  Pith  of  a  field  stem  10  mm.  diameter }•    176 

6.  An  old  leaf-trace 

7.  Outer  cortex  of  a  field  stem 

8.  Outer  edge  of  cortex  and  inner  zone  of  cork  derived  from  collen- 

chyma 

43.  i.  Base  of  1908  growth,  August.     Cedros,  irrigated 

2.  Growth  of  1908  in  April  1909.     Cedros,  irrigated 

3.  Cortex,  2-year-old  stem.    Caopas,  irrigated j-    192 

4.  Pith  of  same  plant 

5.  Epidermis  and  palisade  of  an  old  leaf,  field  plant J 

44.  A.  Irrigated  plant,  2  years  old.     Basal  branches  which  have  rooted  "] 

are  spread  apart j-    192 

B.  Mariola  showing  the  same  behavior,  normal  in  this  species .  .  .  .  J 

45.  A.  Flat  filled  with  paper  tubes,  i  square  inch  in  transverse  section.  ] 

B.  Flat  with  4-square-inch  tubes 

C.  Exp.  141  (3),  i -inch  tubes;  very  poor  growth.    Exp.  143,  4-inch  [   204 

tubes 

D.  The  same,  seedlings  well  grown J 

46.  A.  Seedlings  from  experiments  indicated \ 

B.  Irrigated  plant  (Caopas)  with  a  retono I   2 


GUAYULE  (PARTHENIUM  ARGENTATUM  GRAY): 

A  RUBBER  PLANT  OF  THE  CHIHUAHUAN  DESERT. 


By 

FRANCIS  ERNEST  LLOYD, 

Professor  of  Plant  Physiology,  Alabama  Polytechnic  Institute. 


CHAPTER  1. 
HISTORICAL  ACCOUNT. 

Since  about  the  middle  of  the  last  century,  after  the  epoch-making 
discovery  of  Charles  Goodyear  was  made,  the  demand  for  crude  rubber 
has  been  steadily  increasing.  This  demand  was  for  a  long  period  satis- 
fied by  the  products  harvested  from  the  tropical  forests  of  the  Old 
and  New  Worlds  by  natives  whose  methods  are  resulting  in  a  gradual 
depletion  of  the  natural  supply.  This,  in  turn,  has  stimulated  research 
in  three  directions  :  toward  obtaining  a  synthetic  rubber,  the  ambition 
of  the  chemist;  toward  discovering  other  rubber-producing  plants,  for 
which  search  has  been  made  into  the  farthest  reaches  of  the  tropical 
forests  of  the  world;  and,  finally,  in  the  direction  of  the  culture  of  the 
various  plants  which  before  had  been,  in  their  feral  condition,  the  source 
of  the  much-desired  material.  Whatever  the  promise  of  the  chemist 
may  be,  there  appears  to  be  no  abatement  of  interest  at  present  in  the 
culture  of  those  better-known  trees  which  have  been  found  to  adapt 
themselves  to  the  hand  of  man,  or  in  the  discovery  of  hitherto  unknown 
rubber  plants.  Each  new  announcement,  however  vague  the  authority 
may  be,  that  a  new  rubber  plant  has  been  found,  is  hailed  with  precipi- 
tous interest;  and  one  that  is  well  founded  is  soon  followed  by  a  period 
of  exploitation  scarcely  less  fevered  than  on  the  finding  of  new  gold- 
bearing  fields.  When,  a  very  few  years  ago,  it  became  more  generally 
known  that  the  plant  commonly  known  as  the  guayule,  and  containing 
an  economically  valuable  amount  of  rubber,  grew  in  abundance  in  the 
desert  country  of  northern  Mexico,  the  vegetation  of  the  adjacent  arid 
areas  underwent  minute  examination  in  the  hope  of  finding  either  this 
or  other  similarly  valuable  plants,  and  even  at  the  present  moment  the 
excitement  has  not  died  away. 

The  mere  fact,  however,  that  a  plant  indigenous  to  the  desert 
should  be  found  to  be  of  enough  value  to  set  in  motion  large  business 
operations  involving  millions  of  capital,  based  on  the  amount  of  the 
raw  material  in  sight,  is  sufficient  to  awaken  definite  interest.  The 
economic  value  of  the  desert  is  changed,  and  possibilities  for  the  devel- 
opment of  wealth  in  a  supposedly  barren  country  take  on  new  dimen- 
sions. This  has  occurred,  in  point  of  fact,  as  a  direct  result  of  the  dis- 
covery that  the  plant  guayule  produces  in  the  neighborhood  of  10  per 
cent  of  its  weight  of  "bone-dry"  marketable  rubber.  With  the  eco- 
nomic history,  bionomics,  structure,  and  micro-chemistry  of  this  plant  the 
present  essay  has  to  deal. 

ORIGINAL  DISCOVERY  AND  DESCRIPTION. 

The  guayule  was  first  discovered  by  J.  M.  Bigelow,  M.D.,  in  1852, 
while  attached  to  the  Mexican  Boundary  Survey,  "  near  Escondido  Creek, 
Texas."  It  was  first  described  by  Professor  Asa  Gray  some  years  later. 
His  original  description  was  based  upon  the  type  specimen,  which  is  now 

3 


4  Guayule. 

in  the  Gray  Herbarium  of  Harvard  University.  A  reproduction  of  this 
specimen  is  here  given  (plate  2,  fig.  A) .  The  name  in  the  right-hand  corner 
is  in  the  writing  of  Professor  Gray.  The  label  is  Bigelow's  field  label.  Fol- 
lowing is  the  description  published  in  the  "Botany  of  the  Boundary," 
p.  86,  1859  : 

Parthenium  argentatum  ( sp.  nov. )  :  fruticosum,  pube  brevi  appressima 
sericeo-incanum ;  foliis  spathulato-lanceolatis  oblongisve  in  petiolum  longe  attenu- 
atis  parce  dentatis  seu  laciniatis  sub-triplinerviis ;  ramulis  floridis  elongatis  nudis 
oligocephalis ;  involucri  squamis  obtusissimis ;  acheniis  sericeis;  pappo  e  paleis 
2  membranaceis  lanceolatis. —  Near  Escondido  Creek,  Texas,  in  rocky  places, 
Sept.,  1852;  Dr.  Bigelow. — A  well-marked  species,  connecting  the  sections  Argy- 
rochaeta  and  Parthenichaeta ;  the  leaves  and  branches  whitened  with  a  very  fine 
and  close  silk-silvery  pubescence,  which  appears  to  be  wholly  or  nearly  persistent . 
Leaves  one  to  two  inches  long,  including  the  tapering  base  and  petiole;  2  to  5 
lines  wide,  mostly  acute,  scarcely  veined,  beset  on  each  margin  with  from  one  to 
three  salient  teeth,  or  sharp  lobes.  Flowering  branchlets  slender,  4  to  8  inches 
long,  nearly  leafless  and  peduncle-like,  bearing  3  to  7  sub-sessile  heads  (as  large 
as  those  of  P.  incanunt)  in  a  cluster.  Exterior  scales  of  the  involucre  short,  orbic- 
ular-ovate; the  inner  orbicular,  scarious-membranaceous.  Paleae  of  the  pappus 
lanceolate  or  oblong-lanceolate,  rather  narrower  and  less  obtuse  than  in  P.  hyster- 
ophorus,  puberulent,  the  inner  edge  more  or  less  adnate  to  the  base  of  the  broadly 
obovate  and  cucullate  emarginate  ligule.  l  (Fig.  9.) 

As  will  be  seen,  the  crowding  of  the  heads  to  form  a  "cluster"  de- 
pends upon  external  conditions.  In  a  later  description  published  by 
Gray  in  the  "Synoptical  Flora,"2  we  find  the  first  hint  of  the  peculiarity 
which  later  brought  it  into  economic  prominence.  This  description  is 
as  follows: 

P.  argentatum  Gray.  Suffrutescent,  a  foot  high,  silvery-canescent  with  close 
tomentum;  branches  erect,  rather  leafless  above,  bearing  comparatively  large  and 
few  heads  (of  2  lines  in  diameter) ;  leaves  lanceolate  to  spatulate  in  outline,  some 
entire  or  incisely  2-3  toothed,  the  larger  incisely  pinnatified  into  2  to  7  acute 
lateral  lobes;  pappus  a  pair  of  lanceolate  chaffy  awns  (Bot.  Mex.  Bound.,  86; 
Southwest  border  of  Texas,  Bigelow;  Adj.  Mex.,  Parry,  Palmer;  produces  a  gum 
or  resin  in  Mexico). 

THE  VULGAR  NAME. 

The  name 3  "  guayule  "  is  properly  applied  only  to  Parthenium  argen- 
tatum Gray.  On  account,  however,  of  a  superficial  resemblance  it  has  to 
certain  other  plants,  especially  because  of  similarities  in  size  and  in  the 
gray  color  (so  often  seen  in  the  desert)  of  the  foliage,  these  have  been 
wrongly  called  by  the  same  name.4  The  mariola  (P.  incanum  H.  B.  K., 
plate  44,  fig.  B),  a  closely  related  species,  is  one  of  these;  and  its  very 
general  association  with  the  guayule  proper  has  led  to  much  error  in 
estimating  acreage  of  guayule.  It  is  of  interest  in  this  connection  to  note 
that  the  mariola  is  known  to  the  peon,  in  some  parts  at  any  rate,  as 
"hembra  de  guayule,"  5  apparently  because  of  the  very  constant  associ- 

1  Gray,  in  Torrey,  Botany  of  the  Boundary,  U.S.  and  Mex.  Boundary  Surv., 
p.  86,  1859, 

'Synoptical  Flora  of  North  America,  vol.  i,  pt.  2,  p.  245,  1886. 
'Investigated  by  Endlich,  1905. 

4  The  name  is  also  applied  to  Crysactinia  mexicana  Gray,  and  more  recently 
also  to  Euphorbia  misera,  material  of  which  was   sent  to  Dr.  J.  N.  Rose,  of  the 
U.  S.  National  Herbarium,  from  southern  California,  on  the  supposition  that  it 
contained  rubber. 

5  The  female  guayule. 


A.  The  type  specimen  of  Parthenium  argentatum  Gray. 

B.  Transverse  section  of  a  very  old  stem. 


Encelj 

l»v  ma 


Historical  Account.  5 

ation  of  the  two  species,  and  because  of  the  belief  that  this  association 
is  in  some  way  necessary  to  the  production  of  seed.     Other  species  of 
the  genus,  some  of  which  are  annuals,  have  also  received  the  name  guay- 
ule,  while  a  plant  of  the  Sonoran  Desert  (Sonora  and  southern  Arizona), 
'nceUa  farinosa,  is  not  only  mistaken  to-day  for  guayule  but  is  believed  / 
>y  many  to  contain  rubber.    The  amount,  if  present  at  all,  is  so  insig-// 
nificant  that  it  would  certainly  not  repay  consideration  from  a  com-// 
mercial  point  of  view. 

The  guayule  is  known  also  as  "yerba  de  hule"  in  the  region  of 
Pasaje,  Durango,  and  simply  as  "hule"  in  some  parts  of  Zacatecas  and 
of  Chihuahua.  An  alternative  spelling  "  yule  "  (which  occurs  incorrectly 
as  "Hule"  in  "guallule")  is  used  in  some  parts  of  San  Luis  Potosi.  The 
name  xihuite  l  occurs  in  northern  Zacatecas  and  "about  Saltillo"; 
copallin  and  afinador  are  other  less-used  designations.  But  the  name 
"guayule"  thus  spelled  is  in  the  ascendant  and  will  in  all  probability 
replace  other  names.  Its  derivation,  in  common  with  other  Mexicanisms, 
has  speculative  interest.  Seler  2  would  refer  it  to  quahu  (wood,  tree,  or 
forest)  and  olli  (rubber,  Sp.  hule),  evidently  believing  it  to  be  of  Aztec 
origin.  This  etymology  finds  support  in  the  aboriginal  term  ulequahuitl, 
said  by  Sahagun  (1529)  and  Augustin  Torquemada  (1615)  to  be  applied 
to  a  latex  tree  (probably  Castilloa)  producing  ulli,  a  dark  resin  which 
becomes  very  elastic  (Jumelie,  1903).  By  inversion,  we  have  quahu  +ule. 
The  suggestion  that  the  derivation  is  from  the  Castilian  hay  (there  is) 
and  the  Aztec  olli,  from  which  we  therefore  have  hayolli,  which  becomes 
hayule  and  so  guayule,  can  not  be  seriously  entertained. 

PRIMITIVE  AND  LATER  USES. 

Contact  with  the  country  peon  of  Mexico  reveals  a  great  deal  of 
resourcefulness  in  the  use  of  many  plants.  In  out  of  the  way  places  a 
game  is  played  with  a  small,  very  resilient  ball,  not  purchased  in  the 
market.  It  proves  on  examination  to  be  of  very  pure  rubber,  obtained 
by  communal  mastication  of  the  bark  of  the  guayule.  Altamirano 
(1906)  tells  us  that  country  boys  obtain  rubber  in  a  similar  manner  also 
from  "tatanini,"  a  name  applied,  in  Queretaro,  to  Parthenium  incanum 
and  to  P.  lyratum.  This  custom  dates  back  with  fair  certainty  to  the 
middle  of  the  eighteenth  century,  having  been  noted  by  a  Jesuit,  one 
Negrete.3 

Mr.  W.  H.  Stayton,  formerly  captain  in  the  U.  S.  Navy,  when  on 
duty  in  the  Gulf  of  California,  observed  the  Yaqui  Indians  ashore  playing 
a  game  with  a  ball  about  twice  the  diameter  of  a  baseball.  The  game 
consisted  in  throwing  the  ball  from  hip  to  hip.  It  is  not  unlikely  that 
the  ball  was  made  of  guayule  rubber,  which  could  have  been  obtained 
from  the  country  east  of  the  Sierra  Madre,  or  even  of  rubber  from  tataninf , 


1  From  the  Nahuatl  xihititl,  weed.     This  spelling  is  given  by  Endlich  (he.  cit.). 
"Jihuite"  is  given  in  Zacatecas.     "Gihuete"  occurs  in  a  legal  instrument  drawn 
up  at  Matamoras,  Coahuila,  under  date  of  March  9,  1905,  in  which  also  "hule"  is 
given  as  designating  guayule. 

2  Endlich,  loc.  cit. 

3  According  to  Juan  Fritz,  fide  Endlich,  1906. 


6  Guayule. 

mariola,  or  other  plant.  The  possibility  that  it  came  from  the  South  is, 
however,  not  excluded.  Peter  Martyr  (1569;  published  in  1830),  Saha- 
gun  (1529),  and  Herrera  (1492-1526)  all  speak  of  balls  made  of  rubber 
made  from  latex  trees.1 

There  can  therefore  be  little  doubt  that,  in  common  with  the  manu- 
facture of  mescal,  extraction  of  fibers,  and  like  primitive  industries,  the 
making  of  rubber  balls  from  the  guayule,  just  as  from  latex  plants, 
antedates  the  invasion  of  Mexico  by  the  Spaniard.  It  may  be  mentioned 
in  passing  that  the  method  of  extracting  the  rubber  as  above  noted  is 
analogous  to  the  only  widely  used  modern  method  of  obtaining  the  crude 
rubber  on  a  large  scale,  namely,  by  a  purely  mechanical  process.  The 
rationale  of  this  will  be  seen  beyond.  In  this  connection  a  recent  dis- 
covery of  a  piece  of  rubber  which  is  undoubtedly  of  ancient  origin  on  an 
old  aboriginal  village-site  in  Arizona  is  of  peculiar  interest.  Of  this 
discovery  the  following  account  is  furnished  me  by  Prof.  R.  H.  Forbes: 

The  lump  of  rubber,  a  portion  of  which  I  recently  handed  you,  was  found  in 
December  (or  thereabouts),  1909,  at  the  west  end  of  the  Santa  Cruz  Reservoir  and 
Land  Company's  dam,  14  miles  west  of  Sasco,  Ariz.  Mr.  C.  O.  Austin,  who  was 
present,  states  that  this  ball  of  rubber  was  contained  in  a  small  olla  with  articles  of 
stone  belonging  to  the  older  prehistoric  ruins  of  this  country.  The  find  was  made 
at  about  3  feet  below  the  general  surface  which  was  formed  by  the  off -wash  of  an 
adjacent  low  mountain.  No  traces  of  houses  on  the  present  level  of  the  land, 
according  to  Mr.  Austin,  were  visible.  One  other  ball  of  rubber  was  found  here, 
and  is  now  in  Col.  W.  C.  Greene's  collection  at  Cananea.  I  regard  this  find  as 
genuine,  as  Mr.  Austin  is  familiar  with  Salt  River  Valley  ruins  and  his  statements 
are  confirmed  by  others. 

Microscopic  examination  of  the  specimen  to  which  Professor  Forbes 
refers  throws  doubt  on  the  view  that  it  is  guayule  rubber,  but  a  final 
statement  can  not  at  present  be  made. 

A  record  of  this  kind  would  be  incomplete  without  reference  to  the 
use  of  guayule  as  a  fuel.  On  account  of  its  resin  content,  the  plant 
burns  with  a  fierce,  smoky  flame,  after  the  fashion  of  "fat  pine;"  so 
that  whenever  it  was  available  it  was  invariably  used  as  a  fuel  for  the 
crude  Mexican  adobe  smelters,  ruins  of  which  are  frequently  seen  in  the 
mining  districts.  In  this  way  thousands  of  acres  have  been  depleted 
of  their  guayule,  a  wasteful  process  which  was  quickly  stopped  when 
the  value  of  the  plant  became  known.  It  can  scarcely  be  doubted  that 
many  peculiarities  of  local  distribution  within  restricted  regions  are  due 
to  the  pulling  of  the  guayule  for  fuel.  Thus  a  large  smelter  and  a  num- 
ber of  roasting  furnaces  were  in  operation  at  Cedros,2  the  head  fraction 
of  the  hacienda  of  that  name  lying  to  the  west  of  Mazapil,  for  a  term 
of  years,  and  this  circumstance  is  often  referred  to  by  the  peons  to 
explain  the  absence  of  guayule  in  places  where  it  would  naturally  be 
expected.  The  case  is  analogous  to  the  use  of  walnut  for  fuel  and  fence- 
rails  in  the  early  days  in  the  eastern  United  States. 

1  Jumelle,  1903,  quotes  these  authors  at  length. 

2  According  to  Juan  Robles,  whose  duty  it  was,  in  1856,  to  weigh  the  shrub 
as  it  came  into  the  fundicidn  at  Cedros,  guayule  was  paid  for  at  the  rate  of  18 
centavos  per  carga  (6  arrobas  =  7o  kilos),  or  about  17  pounds  for  i  cent  (gold)! 
The  women  on  Cedros  burned  guayule  in  their  bread  ovens  as  late  as  1894  (fide  G.  R. 
Fleming).     Guayule  shrub  now  fetches  150  pesos  the  ton. 


Historical  Account.  7 

HISTORY  OF  MANUFACTURE. 

Public  attention  was  drawn  to  guayule  rubber,1  apparently  for  the 
first  time  in  1876,  by  an  exhibition  sent  from  Durango  to  the  Centennial 
Exposition  at  Philadelphia  (Pearson,  1907).  In  the  same  year,  accord- 
ing to  the  Mexican  Herald,  the  Natural  History  Society  of  Mexico  took  up 
the  study  of  the  plant  and  reported  the  presence  of  rubber  of  good 
quality  (Delafond,  1908). 

The  first  move  toward  the  utilization  of  guayule  rubber  other  than 
by  the  natives  appears  to  have  been  made  in  1888,  when  a  company, 
the  name  of  which  is  unknown  to  me,  but  probably  the  Mechanical  Rubber 
Co.,  of  Passaic,  New  Jersey,  sent  a  special  agent  to  Mexico  with  instruc- 
tions to  " obtain  a  large  quantity  "  of  "rubber-bark,"  " from  which  it  was 
proposed  to  extract  the  rubber  by  a  process  of  grinding  and  washing." 
According  to  the  account,  the  agent  seems  not  to  have  clearly  understood 
his  instructions,  and  shipped  to  New  York  100,000  pounds  of  the  entire 
shrub!  The  company  in  question  did  not  relish  paying  the  freight  on 
the  wood,  and  this  item  of  expense  deterred  further  investigation. 
However,  the  shrub  was  decorticated,  the  bark  and  twigs  ground  up 
finely,  and  "immersed  in  hot  water  *  *  *  finally  coagulating  the 
rubber  into  one  mass. ' '  The  result  was  an  extraction  of  1 8  per  cent  rubber 
(the  wood  of  course  not  entering  into  the  count),  the  quality  of  which 
was  regarded  as  equal  "to  the  best  grade  of  Centrals,"  and  a  specimen 
was  reported2  to  have  been  in  good  condition  in  1895.  There  seems  to  be 
little  doubt  that  the  "rubber-bark"  referred  to  in  the  preceding  para- 
graph was  guayule,  though  ignorance  of  the  identification  was  confessed. 
However,  the  material  was  collected  at  Hot  Springs  (Aguas  Calientes), 
Chihuahua,  and  was  referred  to  in  a  letter  by  the  local  agent,  who  under- 
took the  collection,  as  "hule."3 

In  this  same  year,  1888  or  thereabout,  a  Mr.  Herbert  Wilson  sent  a 
sample  of  the  rubber  to  England  for  analysis,  and  at  about  this  time 
also  Heir  Juan  Fritz  employed  a  number  of  peons  to  chew  out  a  suffi- 
cient amount  of  the  raw  material  for  examination,  and  this  he  sent 
for  study  to  a  German  chemist,  whose  report  was  a  practical  condemna- 
tion of  the  rubber  as  an  article  of  commerce. 

Shipments  of  crude  shrub  made  to  Hamburg  in  1900  were  treated 
with  caustic  soda  and  small  amounts  of  rubber  thus  recovered  were 
placed  on  the  market.  In  the  following  year  25  or  30  pounds  of  guayule 
rubber  were  sent  to  the  market  from  a  laboratory  which  had  been  estab- 
lished by  Germans  at  San  Luis  Potosi.  The  earliest  efforts  seem  to  have 
centered  here,  so  that  San  Luis  Potosi  may  be  regarded  as  the  birthplace 
of  the  industry. 

The  laboratory  experience  at  San  Luis  Potosi  led  in  1902  to  the 
establishment  of  a  factory  at  Jimulco,  by  Adolf  Marx,  representing  the 

1 1  have  been  unable  to  obtain  a  transcript  from  the  original  records.  An 
anonymous  writer  in  the  India  Rubber  World,  April  10,  1895,  refers  to  this  exhibit 
as  rubber  from  "a  native  plant  of  the  genus  Cynanchum,  of  the  natural  order 
Asclepiadacese,  according  to  Mr.  Fernando  Altamirano." 

1  In  the  India  Rubber  World,  10  :  April,  1895:  "Extraction  of  rubber  from 
minor  plants"  (unsigned),  upon  which  I  base  the  account  in  this  paragraph. 

3  India  Rubber  World,  loc.  cit. 


8  Guayule. 

Compania  Explotadora  de  Caucho  Mexicano,  from  which  factory  rubber 
was  put  on  the  market  for  the  first  time  in  1905.  In  1902,  also,  certain 
American  capitalists  financed  an .  expensive  but  eventually  successful 
series  of  experiments  which  led  to  the  successful  extraction  of  the  crude 
rubber  by  a  mechanical  process  (devised  by  Mr.  Wm.  A.  Lawrence),  and 
two  years  later,  in  1904,  the  first  lot  of  rubber  thus  prepared  was  taken 
by  the  Manhattan  Rubber  Company.  On  December  25,  1904,  50  pounds 
of  crude  rubber,  extracted  by  means  of  the  now  successfully  adapted 
pebble-mill,  were  shipped  to  the  United  States,  over  half  of  the  amount 
being  purchased  by  the  Manhattan  Rubber  Company.  Then  followed 
the  building  of  a  large  factory  at  Torreon  by  the  Continental- Mexican 
Rubber  Company  (plate  3,  plate  4,  fig.  A),  in  which  the  results  of  the 
earlier  experiments  were  used.  This  event  marked  the  beginning  of 
commercial  success  in  the  extraction  of  rubber  from  the  guayule  shrub 
by  the  mechanical  method,  which  has  superseded  all  others,  and  it 
should  be  said  that  this  phase  in  the  development  of  the  industry  is 
almost  entirely  due  to  American  initiative  and  ingenuity. 

From  1905  on  a  large  advance  in  the  outlay  of  capital  followed,  and 
extracting  plants  of  various  sizes  were  established  in  San  Luis  Potosi, 
Saltillo,  Monterey,  and  Gomez  Palacio,  as  well  as  at  Torreon  and  Jimulco. 

Manufacturing  enterprise  has  lately  brought  the  guayule  industry 
into  Texas.  On  September  i,  1909,  a  factory  1  at  Marathon,  Texas,  in 
the  heart  of  the  guayule  area  of  that  State,  began  operations  under 
the  Texas  Rubber  Company.  But  it  should  be  added  that  the  manu- 
facture of  guayule  rubber  had  already  to  some  extent  been  carried  on 
in  the  United  States  and  abroad.  The  extent  of  this  phase  of  the  indus- 
try is  indicated  in  the  total  exportation  of  crude  shrub  from  Mexico, 
the  statistics  for  which  are  given  on  p.  n.2 

At  the  present  writing,  according  to  Mr.  Henry  C.  Pearson,3  the  out- 
lay of  American  capital  alone  in  Mexico  amounts  to  $30,000,000. 

METHODS  OF  EXTRACTION. 

A  brief  statement  of  the  principal  features  in  the  methods  of  extrac- 
tion of  rubber  from  guayule  will  be  of  interest  here,  especially  as  they 
differ  widely  from  nearly  all  hitherto-used  methods  of  preparing  crude 
rubber  from  latex  plants.4  It  must  be  understood  that  the  rationale  of 
the  processes  lies  in  the  fact  that  the  rubber  exists  as  such  in  the  cells  of 
the  plant,  and  will  not  escape  by  bleeding. 

The  material  must,  then,  either  be  dissolved  out,  after  preliminary 
grinding,  by  suitable  chemical  agents,  or  must  be  agglomerated  mechan- 
ically, either  with  or  without  the  assistance  of  a  substance  (caustic  soda) 
which  will  attack  the  cell  wall.  The  chemical  method  is  used  successfully, 
it  is  understood,  at  Akron,  Ohio,  where  an  excellent  brand  of  guayule 

1  Previously  built  and  operated  for  a  short  time  by  the  Big  Bend  Rubber  Co. 

2  We  now  read  that  the  Japanese  have  entered  the  market,  and  are  buying 
shrub  (Dec.,  1909). 

3  India  Rubber  World,  40  :  383,  August  i,  1909. 

*  African  "grass-rubber,"  however,  is  obtained  in  a  crude  way,  but  purely 
mechanically,  from  species  of  Landolphia  (Jumelle,  1903). 


PLATE  3 


A.  Upper  floor  in  a  guayule  factory  from  which  pebble  mills  are  charged. 

B.  Lower  floor:  discharge  chutes  and  ditch  from  pebble  mills. 

C.  A  battery  of  washing  and  sheeting  machines. 


A.  Stacks  of  guayule  in  bales.     Continental-Mexican  Rubber  Co. 

B.  Experimental  ground,  with  plants  two  years  old,  from  stocks.     Cedros. 


Historical  Account.  9 

rubber  is  produced.  Although  the  principles  involved  are  well  known,  the 
precise  steps  are  preserved  secret.  The  process,  which  is  based  on  meth- 
ods of  organic  analysis,  is  not  widely  used,  and  only  a  small  part  of  the 
total  manufacture  is  carried  on  in  this  way. 

Of  greater  interest,  not  only  in  itself,  but  for  the  future  economic 
development  of  the  rubber  industry,  is  the  mechanical  method.  This 
may  be  described  only  in  its  broader  outlines,  since  the  steps  employed 
by  various  manufacturers  are  changed  from  time  to  time  as  experience 
indicates. 

The  shrub  is  first  washed  so  as  to  free  it  from  dust  and  other  foreign 
matters  which  affect  the  specific  gravity  of  the  "worm  "  rubber  by  becom- 
ing attached  to  the  agglomerated  particles.  It  is  then  passed  between 
rolls  which  comminute  it  while  it  is  being  sprinkled  with  water.1  The 
rolls  used  have  been  supplied  with  knives,  or  have  been  adapted  to 
pulverize  the  material,  or,  as  now  used,  the  shrub  may  be  run  twice  be- 
tween corrugated  rollers,  running  differentially,  for  the  sake  of  even  and 
fairly  fine  grinding.  The  resulting  mass  is  then  placed  in  a  pebble-mill, 
which  is  a  short  cylinder  of  steel,  lined  with  Belgian  flint  bricks,  such  as 
is  used  for  grinding  cement,  paint,  charcoal,  and  the  like  (plate  3,  figs. 
A,  B).  The  grinding  is  accomplished  by  means  of  Norwegian  or  Medi- 
terranean flint  shore-pebbles.2 

The  pebble-mill  charge  consists  of  one-third  its  volume  of  pebbles, 
one-half  of  water,  together  with  6  to  8  bushels  of  shrub.  The  mill  is 
revolved  at  the  rate  of  about  30  times  a  minute  for  a  period  lasting  90 
minutes  to  2  hours,  at  the  expiration  of  which  time  there  results  a  finely 
ground  pulp  consisting  of  minute  agglomerations  of  rubber  mixed  with 
fine  particles  of  bagasse.  This  is  separated  from  the  dirty  water  in  which 
it  was  ground  and  is  then  run  into  tanks,  where  a  skimming  process  sepa- 
rates the  rubber,  which  floats,  from  the  bagasse,  which  sinks.  A  part 
of  the  bagasse,  however,  does  not  sink  at  this  time,  namely,  that  con- 
sisting of  flakes  of  light  yellow  cork. 

Nor  is  the  rubber  free  from  particles  of  wood  fiber,  imprisoning  more 
or  less  air,  and  this  interferes  with  the  complete  separation  of  rubber  and 
bagasse.  The  complete  water-logging  of  the  bagasse  may  be  attained 
by  means  of  a  compressor,  in  which  the  skimmed  rubber,  with  its  adhe- 
rent fiber,  is  subjected  under  water  to  a  pressure  of  about  225  pounds 
for  a  period  of  15  minutes  to  2  hours,  according  to  the  kind  of  shrub 
being  treated.  Subsequent  treatment  in  a  beater-washer,  an  elliptical 
tank,  supplied  with  a  paddle-wheel  of  half  its  transverse  diameter, 
prepares  for  the  final  separation  of  rubber  and  bagasse  in  settling-tanks. 

1  It  has  been  suggested   (Whittelsey,   1908)  that  decortication,  previous  to 
grinding,  would  be  an  economy.     It  is  interesting  to  recall  that  this  was  done — 
on  an  experimental  scale,  albeit  a  generous  one — in  1895   (India  Rubber  World, 
April   1895). 

An  alternative  method,  recently  proposed  by  Chute  and  Randel  (India  Rubber 
World,  vol.  42,  p.  360,  1910),  consists  in  grinding  the  shrub  dry  and  then  deresinat- 
ing  (the  solvent  to  be  recovered  by  distillation).  The  ground  shrub,  now  supposedly 
free  from  resin,  is  then  treated  as  here  described,  beginning  with  the  pebble-mill. 

2  The  internal  structure  of  this  mill  has  been  the  subject  of  numerous  patents. 
Thus,  steel  balls,  associated  with  various  forms  of  projections  from  the  interior 
surface  of  the  cylinder,  have  been  used,  but  without  supplanting  the  "  pebble-mill." 


10  Guayule. 

An  alternative  treatment  consists  in  allowing  the  washed  rubber 
from  the  first  skimming-tank  following  the  pebble-mill  to  soak  for  a  week 
in  settling-tanks,  during  which  time  the  bagasse  becomes  water-logged 
and  sinks.  The  soaking  is  probably  of  value  also  in  separating  from  the 
rubber  certain  substances,  probably  enzymatic  in  character,  which  other- 
wise would  contribute  to  the  earlier  breaking  down  of  the  rubber. 

The  clean  rubber  is  now  passed  between  corrugated  and  smooth 
rolls  for  the  purpose  of  washing  and  sheeting  (plate  3,  fig.  C),  when  the 
product  is  ready  to  be  put  on  the  market.  Unless  further  treatment 
ensues,  the  rubber  thus  prepared  contains  about  25  per  cent  moisture, 
together  with  a  proportion  of  resin.1 

Other  special  steps  in  treatment  are  applied  to  the  separation  of 
rubber  from  bagasse,  or  in  preparing  special  grades.  For  example, 
boiling  the  skimmed  rubber  in  a  i  to  2  per  cent  solution  of  caustic  soda 
has  been  used  as  an  aid  in  the  separation  of  rubber  and  fiber,  and  for 
partial  deresination  by  the  saponification  of  the  resin  acids.  By  this 
means  the  amount  of  resin,  25  per  cent,  usually  present,  may  be  reduced 
to  17  or  1 8  per  cent.2  Other  modifications  in  treatment  are  necessitated 
by  the  condition  of  the  plants  when  treatment  is  begun.  Old,  weathered 
and  dried-out  shrub  is  not  worked  with  the  same  ease  nor  with  the  same 
result  as  fresh,  while  a  certain  amount  of  seasoning  is  an  advantage.  Con- 
siderable losses  have  been  entailed  by  storing  guayule  in  the  yard  exposed 
to  the  sun  (plate  4,  fig.  A),  as  may  be  imagined  if  a  million  dollars'  worth 
of  shrub  is  handled  in  this  way,  even  though  the  amount  of  deterioration 
is  small.  This  loss  is  now  avoided  by  placing  the  shrub  in  storehouses. 

THE  NATURAL  SUPPLY  OF  SHRUB. 

With  such  large  interests  at  stake,  it  soon  became  a  matter  of 
moment  to  determine  the  relation  of  supply  of  the  shrub  to  the  manu- 
facture, as  to  total  supply  in  sight,  as  to  its  rate  of  reproduction  under 
natural  conditions,  and  as  to  the  possibility  of  its  cultivation. 

The  first  of  these  questions  was  naturally  the  first  to  be  raised,  and 
many  attempts  have  presumably  been  made  to  find  an  answer.  The 
earliest, and,  so  far  as  I  am  aware, the  only  published  calculation  was  made 
by  Endlich  (loc.  «'*.), who  assumed  an  average  amount  of  half  a  ton  per 
hectare  in  virgin  fields.  The  total  area  of  the  general  guayule  region 
being  taken  as  75,000  square  kilometers,  and  assuming  that  only  one- 
tenth  of  this  carries  the  shrub,  Endlich  arrived  at  the  sum  of  375,000 
tons  total  supply  in  Mexico,  which,  at  the  rate  of  7  to  10  per  cent  of 
rubber,  represents  26,250  to  37,500  tons  of  rubber.  This  estimate  was 
probably  quite  conservative,  as  indicated  by  calculations  based  upon 
official  reports  brought  together  in  the  India  Rubber  World. 

Using  the  probable  corrections  for  exports  of  crude  rubber  other 
than  guayule,  this  publication  gives  the  total  imports  of  guayule  rubber 

1  Whittelsey,  1909. 

1  At  this  writing  an  announcement  is  made  (Guayule  Rubber  by  a  New  Proc- 
ess, India  Rubber  World,  December,  1909)  that  a  method  ("  physico-mechanical " 
— sic.)  has  been  patented  whereby  crude  rubber,  after  treatment,  has  the  com- 
position: "Pure  caoutchouc.  88  per  cent;  resin,  7  per  cent;  water,  5  per  cent." 


Historical  Account.  11 

into  the  United  States  from  June  30,  1905,  to  June  30,  1909,  as  32,010,820 
pounds.  This  being  about  80  per  cent  of  the  total  export,  using  the 
data  for  1906-1908  *  as  a  basis,  we  have  a  total  exportation  of  crude 
guayule  rubber  for  four  years  of  40,013,525  pounds,  which  amount  to 
20,000  long  tons  in  round  numbers,  representing,  on  the  basis  of  7  per 
cent  extraction  of  rubber  with  25  per  cent  moisture  (5.25  per  cent  dry 
rubber),  shrub,  286,000  tons;  and  on  the  basis  of  15  per  cent  extraction 
of  rubber  with  25  per  cent  moisture  (11.25  Per  cent  dry  rubber),  shrub, 
132,000  tons. 

The  last  two  sums  give  us  the  highly  probable  extremes  between 
which  the  tonnage  of  shrub  represented  by  crude-rubber  exports  falls. 
To  the  amount  must  be  added  the  amount  of  shrub  exported,  for  which 
figures  for  two  and  a  half  years  are  available,  namely,  2745  tons.  We 
have,  therefore,  the  limits  of  288,745  tons  and  134,745  tons. 

That  the  larger  amount  of  shrub  is  nearer  the  true  amount  taken 
appears  to  be  the  case,  since  the  extraction  of  rubber  with  25  per  cent 
moisture  has  only  recently  reached  15  per  cent,  and  this  is  probably 
not  attained  by  manufacturers  in  general.  For  a  long  time  it  fell  below 
10  per  cent,  so  that  an  average  extraction  of  8  to  10  per  cent  of  rubber 
(25  per  cent  moisture)  is  probably  near  the  truth.  This  would  represent, 
on  the  8  per  cent  basis,  252,745  tons  shrub;  on  the  10  per  cent  basis, 
202,745  tons  shrub. 

It  is  therefore  probable  that  in  the  neighborhood  of  225,000  tons 
of  shrub  were  disposed  of  up  to  June  1909.  This,  according  to  Endlich, 
would  be  somewhat  over  half  the  total  original  available  supply.  This 
estimate  agrees  with  that  of  some  interested  informed  persons  who  hold 
that  one-half  of  the  original  supply  is  used.  But  estimates  carefully  made 
for  business  purposes  show  that  there  were  at  this  time  at  least  200,000 
tons  still  available.  Of  this  amount,  I  myself  have  seen  at  least  100,000 
tons  in  a  comparatively  restricted  area  on  three  estates. 

Allowing  for  guayule  still  remaining  on  fields  which  have  been  gone 
over,  and  which  in  certain  well-known  cases  is  in  considerable  amount, 
it  seems  not  improbable  that  the  total  original  amount  reached  500,000 
tons.  The  amount  in  Texas  in  the  Big  Bend  country  is  not  known  and 
must  therefore  be  left  out  of  account,  but  without  it  it  does  not  seem 
probable  that  the  total  amount  of  virgin  shrub  is  sufficient  to  last  more 
than  four  to  six  years  at  the  present  rate  of  consumption.2  It  is  likely 
that  the  smaller  concerns  will  be  closed  out,  so  that,  with  a  reasonably 
restricted  output,  the  supply  may  be  made  to  last  six  to  eight  years, 
which  is  the  period  during  which  the  solution  of  the  cultivation  of  the 
plant  must  be  compassed  if  it  is  to  keep  the  industry  on  its  feet. 

1  India  Rubber  World,  September  1909.     For  1906  to  1908  the  total  crude 
rubber  exports  were  22,693,489  pounds,  while  our  total  imports  were  17,917,342 
pounds. 

2  The  recent  high  prices  paid  for  crude  rubbers  have  stimulated  the  manu- 
facture of  guayule  rubber,  which  has  brought  as  much  as  $1.25  per  pound.     The 
imports  into  the  United  States  for  the  year  ending  June  1910  were,  approximately, 
10,000  long  tons.     On  the  basis  stated  above,  this  quantity  represents  something 
between  66,000  and  145,000  tons  of  shrub,  but,  in  view  of  the  improved  methods, 
the  smaller  figure  lies  nearer  the  truth.    If  we  assume  a  1 2  per  cent  extraction,  we 
get  83,300  tons  of  shrub  used  in  the  year. 


1 2  Guayule. 

As  in  all  commercial  enterprises  depending  upon  the  rate  of  growth 
of  the  raw  material,  and  more  notably  of  lumber  trees,  the  methods 
were  and  still  are  conducted  without  relation  to  the  future.  When, 
however,  the  capitalist  began  to  see  that  nature  had  set  a  definite  limit 
to  the  rate  of  supply,  it  became  a  matter  of  moment  to  determine  what 
could  be  done  to  meet  the  demand.  The  method  of  obtaining  the  shrub, 
when  not  owned  outright,  is  by  contract  between  the  companies  and  the 
hacendados  whose  lands  support  a  growth  of  the  desired  plant.  These 
gentlemen  at  first  signed  contracts  at  a  very  low  figure,  but  when  they 
saw  the  market  stiffen  and  their  acreage  continually  reduced,  they  very 
naturally  began  to  take  thought  for  the  morrow.  I  have  conversed 
with  hacendados  who  had  for  some  years  endeavored  to  germinate  the 
seed,  in  the  hope  of  solving  the  problem  of  cultivating  the  plant.  Lack 
of  success,  however,  was  the  chief  result  of  such  effort,  though  a  few 
doubtless  succeeded  in  getting  plants  to  grow.  Indeed,  optimistic  state- 
ments as  to  the  possibility  of  growing  the  plant  profitably  have  been 
made  in  some  quarters,1  and  it  has  even  been  claimed  that  the  whole 
problem  of  cultivation  at  a  profit  has  been  solved.  As  will  be  seen,  how- 
ever, in  what  follows,  as  regards  the  secretion  of  rubber,  which  is  the 
all-important  point,  a  very  great  deal  of  caution  should,  in  view  of  the 
lack  of  evidence,  have  qualified  any  statement  of  this  kind.  It  seems 
more  consonant  with  the  truth,  as  well  as  with  business  methods  (a 
not  invidious  juxtaposition,  it  is  hoped),  to  take  a  skeptical  attitude, 
which,  however,  need  not  be  unduly  pessimistic.  It  is  rash  at  best  to 
attempt  to  foretell  what  solution  science  may  bring  to  any  problem. 

ATTEMPTS  AT  CULTURE. 

That  hope  has  been  entertained  that  the  cultivation  of  guayule  on 
a  profitable  basis  may  be  possible  is  evident.  In  addition  to  private 
owners,  at  least  two  companies  have  spent  time  and  money  in  seeking 
this  end,  if  unauthoritative  statements  may  be  relied  upon.  Of  these 
the  Continental-Mexican  Rubber  Company  essayed  to  make  a  serious 
trial,  and  employed  a  scientific  corps  to  undertake  research  looking  to 
the  final  solution  of  the  question.2 

It  is  not  surprising  that  so  valuable  a  desert  plant  should  have 
attracted  the  attention  of  interested  persons  of  other  nations  whose 
authority  extends  over  desert  areas  in  other  parts  of  the  world.  No 
detailed  statement  on  this  score  can  be  made,  however,  beyond  that  the 
Germans  3  are  said  to  be  conducting  experiments  in  the  cultivation  of 
guayule  in  East  Africa.  The  feeling  properly  exists  that  any  effort 
toward  the  subjugation  of  the  desert  is  justified.  The  time  will  come 
when  not  only  those  parts  of  arid  regions  which  may  be  brought  under 
irrigation,  but  those  also  which  remain  unmodified  in  this  regard,  will 
yield  their  possibilities  to  the  hand  of  man,  and  we  stand  at  this  moment 
at  the  serious  beginning  of  this  conquest. 

1  See  India  Rubber  World,  May  i,  1908. 

2  This  work  has  recently  been  taken  up  anew  (September  1910). 

3  Ross,  1908. 


CHAPTER  II. 

THE  ENVIRONMENT. 

GEOGRAPHICAL  DISTRIBUTION. 

The  northern  limit  of  distribution  of  the  guayule  is  in  the  southwest- 
ern part  of  Texas,  where  it  occurs  in  Presidio,  Brewster,  and  Pecos  (near 
Langtry)  Counties.  This  area  is  continuous  with  its  area  of  distribu- 
tion in  Mexico,  throughout  which  it  occurs  with  greater  or  less  frequency. 
The  periphery  of  this  area  runs  approximately  as  follows:  from  the  west- 
ern extremity  of  Presidio  County  in  Texas,  the  western  boundary  will  run 
somewhat  west  of  south  till  it  reaches  the  northern  boundary  of  Du- 
rango,  near  Santa  Barbara,  Chihuahua.1  From  this  point  the  limit  turns 
approximately  toward  the  southeast,  running  parallel  with  the  Mexican 
Central  Railway  at  a  distance  of  about  100  kilometers  (Endlich,  1905). 
Beyond  the  state  of  Durango  the  boundary  turns  still  farther  to  the  east, 
curving  northward  again  not  far  from  the  city  of  San  Luis  Potosi.2  The 
loist  meridian  marks  roughly  the  eastern  boundary,  lying  somewhat  west 
of  it  till  beyond  Saltillo,  where  the  boundary  then  curves  slightly  west  of 
north,  reaching  the  eastern  limit  in  Texas  at  about  Langtry.  The  north- 
ern limit  is  marked  approximately  by  Fort  Stockton. 

The  guayule  is  thus  seen  to  be  peculiar  to  the  Chihuahuan  desert. 
The  belief  which  has  sometimes  been  entertained  that  it  occurs  in  western 
Sonora,  southern  Arizona,  and  New  Mexico  seems  not  to  be  well  founded, 
and  the  area  within  which  it  is  found  is  confined  to  the  northern  portion  of 
the  central  plateau,  embracing  an  area  of  approximately  130,000  square 
miles,  or  290,000  square  kilometers.  Of  this  area,  it  will  be  understood 
that  only  a  small  proportion  will  be  found  to  carry  guayule,  and  a  rough 
estimate  of  10  per  cent  would  certainly  not  be  too  low.  Endlich 's  (1905) 
estimate,  75,000  square  kilometers,  is  probably  as  nearly  correct  as  we 
may  make  it.  It  may  here  be  remarked  that  the  very  great  irregularity 
of  distribution  makes  it  very  difficult  indeed  to  make  anything  approach- 
ing an  accurate  estimate  of  the  amount  of  guayule  as  to  acreage  alone, 
aside  from  the  question  of  density,  so  that  any  figures  which  may  be  given 
are  subject  to  correction. 

ALTITUDINAL  DISTRIBUTION. 

The  whole  region  in  question  is,  as  already  said,  embraced  within 
the  northern  part  of  the  central  plateau  (mesa  central)  of  Mexico  and 
the  adjacent  area  within  which  guayule  is  found  in  Texas.  This  area 
has  an  altitude  varying  from  2,000  to  10,000  feet  above  sea-level.  The 

1  Mr.  W.  H.  Stay  ton  reports  seeing  a  small  amount  of  guayule  in  the  Sierra 
Madre  east  of  Sahuaripa,  Sonora.  The  amount  on  the  eastern  slope  was  somewhat 
greater  than  on  the  western.  It  is  now  believed  to  occur  sparingly  in  eastern  Sonora. 

2 1  am  informed  that  Pringle  found  guayule  near  Pachuca,  Hidalgo,  which  is 
probably  its  southernmost  limit. 

13 


14  Guayule. 

range  of  the  plant  in  altitude  extends  from  the  lower  limit  mentioned 
to  about  7,000  feet,  or  somewhat  higher.  As  observed  by  Endlich  (1905), 
however,  the  most  important  acreage  is  not  usually  to  be  found  much 
above  6,000  or  6,500  feet. 

CLIMATE. 

The  climatic  conditions  under  which  the  guayule  lives  have  not 
only  scientific  interest,  but  very  important  practical  bearings  as  well. 
This  will  be  understood  upon  the  reflection  that  many  proposed  opera- 
tions relative  to  the  culture  of  the  plant  involve  the  use  of  water,  and 
whatever  the  theoretical  possibilities  may  be,  success  on  a  large  scale 
must  be  conditioned  very  closely  by  the  nature  of  the  desert  areas  to 
be  utilized.  The  details  in  question  will  be  considered  in  Chapters  VIII 
and  IX.  For  these  reasons  a  somewhat  detailed  account  of  the  actual 
climatic  conditions  observed  at  Cedros,  in  North  Zacatecas,  will  be  given. 

RAINFALL. 

Fortunately,  perhaps,  for  our  purposes,  the  year  (1907-08)  during 
which  observations  were  begun  was  unusually  dry,  and  afforded,  we 
believe,  about  the  most  rigorous  conditions  which  the  vegetation  is 
subjected  to  without  marked  unfavorable  results.  It  is  to  be  regretted 
that  data  for  the  whole  of  this  year  can  not  be  reported,  since  observa- 
tions could  not  be  commenced  before  the  month  of  August.  Relying 
upon  estimates  and  upon  general,  verbal  reports,  and  judging  by  analogy 
with  the  region  about  the  city  of  Zacatecas,  where  the  precipitation  for 

1907  was  about  half  (320  mm.)  of  the  mean  for  29  years  (596  mm.),1  it 
seems  reasonable  to  believe  that  the  total  rainfall  for  1907  was  not 
greater  than  175  mm.  (7  inches),  of  which  138  mm.  were  recorded  in- 
strumentally  as  falling  during  the  last  four  months.    The  growing  season, 
as  would  be  indicated  by  the  scant  amount  of  rain  which  fell  earlier  in 
the  year,  was  a  practical  failure  as  regards  crops  in  general,  and  the  indi- 
cations of  growth  in  the  guayule,  which  at  this  moment  concern  us  most, 
were  consonant  with  the  precipitation,  which  was  at  best  very  scanty. 

As  will  be  seen  upon  examination  of  table  i  and  fig.  3,  the  rainfall  for 

1908  was  somewhat  over  10  inches,  which  appears  to  be  about  normal, 
while  the  effective  rains  fall  in  the  summer  months.     In  1908  it  was  suf- 
ficient to  produce  a  prolonged  period  of  relatively  high  atmospheric 
humidity,  while  the  replenishment  of  the  store  of  water  in  the  soil  was 
marked  enough  to  produce  very  pronounced  mesophytic  conditions.     In 
the  low-lying  flats,  especially  where  the  more  abundant  collections  of  water 
were  formed,  annual  plants  of  weedy  appearance  grew  densely  breast- 
high,  and  seedlings  of  the  mariola  scattered  among  them  grew  with  great 
rapidity  to  a  height  of  40  to  50  cm.  in  one  season.    On  the  low  ridges  and 
in  the  hills  the  available  stratum  of  the  soil  was  full  of  water,  and  the 
guayule  and  mariola,  together  with  many  other  shrubs  and  annuals, 
were  in  full  bloom  and  making  rapid  growth  in  June.     Other  features 
of  the  distribution  of  rains  are  indicated  in  a  general  way  in  the  diagram 
and  are  of  importance  as  related  to  the  period  of  growth  of  the  guayule, 
to  be  referred  to  beyond. 

1  Boletin  Mensual  del  Obs.  Astron.-Meteor.  Zacatecas,  Jan.  21,  1908. 


The  Environment.  15 

TABLE  i. — Rainfall  at  Cedros,  September  1907  to  August  1908  (fig.  3). 


Date. 

Millimeters. 

Date. 

Millimeters. 

Date. 

Millimeters. 

1907 

1908 

1908 

Sept.    9 

24.4 

Mar.     9 

trace  ] 

July    7 

9-81 

Oct.      4 
10 

IQ 

68.3  } 

1.2    ^76.7 

7.2  J 

IS 

21 
27 

S3  «•• 

trace  J 

9 
13 
14 

3-0 

12.8 

24.0 

*  y 
Nov.  28 
Dec.     2 

16.2 

3.9] 

Apr.     3 

9 
1  1 

9-4 
trace 
7.0 

20 
21 
22 

5-8 
3-4 
trace 

81.6 

9 
13 

7.8     21.3 
9.6  J 

'7 
May     i 

trace 
3-0 

;i 

29 

4.8 
9.6 
trace 

1908 

18 
27 

3-6 
6.0 

•29.4 

3° 
31 

trace 
8.4. 

Jan.      3 
18 

2  I 

trace 
trace 
trace 

30 
June     i 

16.8 
i  . 

Aug.     i 
6 

4-5  ' 
3-o 

26 

trace 

4 

22 

2  . 

I9- 

43-2 

7 

12 

1-5 

5-o 

34-5 

Feb.     i 

trace 

23 

14- 

18 

2.4 

6 

trace 

25 

6. 

20 

17-7  , 

NOTE.  —  It  seems  probable  that  the  rainfall  for  the  four  months  of  1907  was 
relatively  high,  and  includes  an  amount  which  normally  would  have  been  distrib- 
uted earlier  in  the  year,  that  is,  in  the  summer  months. 


I  visited  Cedros  during  April  1909.  Upon  arrival  there  it  was  found 
that  there  had  been  no  rain,  save  a  few  drops  on  a  few  occasions,  be- 
tween August  20,  1908,  and  April  5,  1909.  On  the  latter  date  heavy 
showers  occurred  over  considerable  areas,  leaving  water  standing  in 
"charcos"  for  several  days.  This  was  a  very  persistent  drought,  and  it 
was  found  to  have  affected  guayule  quite  unfavorably  in  many  localities. 
I  am  informed  by  Mr.  G.  R.  Fleming  that  drought  again  persisted  till 
June  1  6,  1909,  when  it  was  broken  and  a  very  abundant  rainfall  ensued 
during  the  summer  of  1909. 

AIR-TEMPERATURES. 

Table  2  shows  observed  temperatures  at  Cedros  during  the  time 
indicated.  The  lacunae  observable  in  May,  June,  July,  and  August  are 
not  as  fatal  to  an  adequate  notion  of  the  prevailing  temperatures  as 
might  be  supposed.  A  brief  study  of  the  table  as  a  whole  will  show  that 
the  temperatures  are  remarkably  uniform,  and  this  is  especially  true  of 
the  months  for  which  data  are  lacking.  The  readings,  therefore,  which 
were  made  nearly  every  day,  were  not  recorded  except  as  they  showed 
variations  of  several  degrees. 

The  lowest  temperatures  to  which  guayule  may  be  subjected  are 
not  known.  The  minima  at  Cedros  are  undoubtedly  higher  than  those 
which  occur  in  the  guayule  region  of  Texas,  but  as  meteorological  data 
for  that  region  are  lacking  we  are  compelled  to  judge  by  those  of  El 
Paso,  the  nearest  station.  The  minimum  temperatures  observed  here 
during  the  last  twenty  years  range  close  to  zero,  so  that  we  may  infer 
that  the  guayule  plant  can  withstand  lower  temperatures  than  those 


16 


Guayule. 


encountered  in  North  Zacatecas.1  Attempts  which  may  be  made  in  the 
future  in  the  cultivation  of  the  plant,  e.g.,  in  New  Mexico,  must  be  made 
with  regard  to  its  resistance  to  cold,  and  it  is  to  be  regretted,  therefore, 
that  a  final  datum  on  this  point  can  not  be  given. 


FIG.  i. — Maximum  and  minimum  day  and  night  temperatures  by  months;  maximum 
summer  and  minimum  winter  soil-temperatures  at  10  cm.  depth.     Cedros. 

It  will  be  noted  upon  examining  fig.  i  that  growing  temperatures, 
though  sometimes  low,  occur  even  during  the  winter  months  in  the  day- 
time. At  night,  however,  the  air-temperatures  are  seen  to  be  practically 
non-effective  between  the  middle  of  September  and  the  beginning  of 
May.  This  condition,  judging  from  air-temperatures  alone,  may  be 
regarded  as  resulting  in  a  functional  resting-period  of  at  least  three 
months;  that  is,  the  amount  of  growth  possible  in  the  year  would  be 
that  occurring  within  nine  months  of  time,  aside  from  the  considera- 
tion of  rainfall.  The  soil-temperatures  are  of  course  higher,  and  are,  on 
account  of  the  high  insolation,  frequently  favorable  for  the  absorption 
of  water  by  the  roots,  which  would,  under  favorable  conditions  of  soil- 
moisture,  be  important  in  respect  to  the  water-content  of  the  plant, 
though  it  might  not,  except  when  water  was  abundant  or  under  other- 
wise exceptional  conditions,  stimulate  growth.  The  conditions  as  re- 
gards growth,  then,  may  be  stated  thus:  The  winter,  or  resting-period, 
is  effective  during  the  night-time  chiefly  during  October  and  on  to  the 
end  of  April.  The  day  temperatures  during  this  period  may  effect  growth 
when  water  is  sufficient. 


1  We  now  have  records  showing  that  guayule  can  stand  a  temperature  of  5°  F. 
at  Marathon,  Texas,  and  of  10°  F.  at  Tucson,  Arizona. 


The  Environment. . 


17 


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18  Guayule. 

TABLE  3. — Maximum  and  minimum,  temperatures  at  Cedros. 


Month. 

Max. 

Min. 

Mean. 

Month. 

Max. 

Min. 

Mean. 

•F. 

°F. 

°F. 

°F. 

•F. 

°F. 

Sept.... 

82 

64 

72 

March.  .  . 

98 

32 

71-4 

Oct  
Nov  

86 
8s 

53 
39 

68 
60 

April  .... 
May 

97 

IOO 

47 
40 

71.9 

74 

Dec  
Jan  
Feb  

85 
86.6 

9i 

26 

22.8 

30 

55-9 
57-7 
63.8 

June  
uly.... 
ug  

IOO 

95 
90 

55 
5° 

53 

75 

ji 

SOIL-TEMPERATURES. 

A  single  record  of  soil-temperatures  extending  over  a  period  of  15 
months  was  made  by  means  of  a  standard  pair  of  thermometers.  The 
instruments  were  buried  at  a  depth  of  10  cm.1  below  the  surface  of  the 
ground  at  station  3.  The  surface  had  a  gentle  slope  toward  NE.  by  E., 


50° 


\ 


FIG.  2. — Air-temperatures  (*.)  and  soil-temperatures  at  2  cm.  depth.     November  6,  1907. 

and  would  therefore  receive  neither  the  highest  maximum  nor  the  lowest 
minimum  insolation.  The  indices  stood  at  70°  F.  when  the  instruments 
were  buried,  on  December  31,  1907.  They  were  removed  April  2,  1909, 
when  the  following  readings  were  taken:  maximum  temperature,  94°  F. ; 
minimum  temperature,  52°  F. 


1  This  is  about  the  average  depth  at  which  the  lateral  roots  of  the  guayule 
are  placed. 


The  Environment.  19 

It  is  seen,  therefore,  that  at  the  depth  mentioned  the  lower  critical 
growth  temperatures  in  the  soil  are  probably  never  reached,  and  it  is 
to  be  inferred  that  the  dormant  condition  of  the  vegetation  is  deter- 
mined by  other  factors,  namely,  soil-moisture  and  air-temperatures,  and 
of  these  the  factor  of  moisture  is  probably  the  more  effective. 

The  temperatures  affecting  germination,  however,  are  those  of  the 
surface  of  the  soil  or  at  a  very  slight  depth.  Fig.  2  presents  the  curves  of 
air  and  soil  temperatures  for  November  6,  1907,  at  a  time  when  difficulty, 
ultimately  shown  to  be  due  to  other  causes  than  temperature,  was  ex- 
perienced in  germinating  seeds  in  boxes.  The  soil-readings  are  for  a  depth 
of  2  cm.,  and  the  soil  was  wet,  but  was  exposed  to  full  insolation. 

The  temperatures  from  about  9  a.m.  till  10  p.m.  can  not  be  said  to 
be  unfavorable,  though  their  effect  upon  the  rate  of  germination  and 
subsequent  growth  would  be  offset  by  the  succeeding  hours  of  cool  soil.1 
The  cooler  period  is  more  marked  during  the  succeeding  months  till 
March  or  April  (fig.  i).  Inasmuch,  however,  as  the  night  temperatures 
are  scarcely  ever  favorable  for  germination  (assuming  40°  Fahr.  as  the 
lower  limit)  before  June  or  after  October,  and  even  during  this  period 
not  especially  so,  we  may  conclude  that  the  existing  temperature  condi- 
tions at  Cedros  are  of  subsidiary  importance  in  determining  the  time  of 
the  year  when  germination  occurs.  This  conclusion  is  supported  by  the 
success  attending  germination  tests  made  in  January  (Kirkwood,  1910), 
when  the  temperatures  ranged  from  32°  to  64°  Fafrr.  At  these  tempera- 
tures, germination  did  not  begin  so  soon  as  when,  later  on,  they  were 
somewhat  higher.  It  therefore  may  be  concluded  that,  aside  from  a  cer- 
tain rhythm  which  may  be  detected,  winter  dormancy  both  in  the  mature 
plant  and  in  the  seed  is  due,  in  the  area  we  are  considering,  rather  to  lack 
of  soil-moisture  than  to  unfavorable  soil-tempefratures.  This  conclusion 
can  not,  however,  be  applied  throughout  the  whole  of  the  guayule  region, 
since  the  winter  temperatures  in  Texas,  are  much  more  unfavorable. 

SOIL-MOISTURE. 

The  residual  soil-moisture  during  sustained  periods  of  drought  may 
be  reduced  to  a  point  below  the  minimum  necessary  to  sustain  life.  This 
is  the  chief  cause  of  the  local  dying  off  of  guayule  during  such  periods. 
Generally,  however,  the  amount  of  soil-moisture,  though  insufficient  to 
stimulate  growth  even  if  other  conditions  are  favorable,  is  more  than 
enough  to  sustain  life,  and  indeed  may  be  enough  for  growth  when  the 
equilibrium  between  the  plant  and  the  environment  is  destroyed.  The 
results  of  certain  experiments  detailed  beyond  show  this  to  be  true. 
Plants  at  station  2,  quadrat  2,  were  pollarded  in  November  1907,  about 
5  to  8  cm.  above  the  surface  of  the  soil,  and  these  had  made  a  marked 
growth  by  February  18,  1908,  although  the  surrounding  plants  showed 
no  growth  at  all,  and  indeed  did  not  until  much  later  on.  While  there 
had  been  a  very  small  amount  of  rain,  it  was  quite  insufficient  to  account 
for  the  growth,  even  in  the  pollarded  plants,  during  the  period  between 
the  dates  above  mentioned.  We  may  therefore  conclude  that  usually 

1  Abbe,  C.,  1905,  p.  36. 


20  Guayule. 

during  dormant  periods  the  soil-moisture  is  considerably  above  the  neces- 
sary minimum,1  but  insufficient  to  stimulate  to  growth,  although,  on 
account  of  lack  of  facilities,  a  quantitative  statement  can  not  be  made. 
This  is  to  be  regretted,  because  the  peculiar  distribution  of  the  guayule 
in  the  foot-slope,  while  Parthenium  incanum  extends  beyond  its  limits 
into  the  play  a,2  is  probably  connected  either  with  the  superior  water- 
holding  capacity  of  the  soil  of  the  foot-slope  or  with  its  greater  air- 
content,  aside  from  the  differences  observable  in  the  topography  of  the 
root-systems  of  these  plants.  The  naked  statement  that  the  guayule 
is  confined  to  slopes  which  are  well  drained3  conveys  little  of  explanation. 


RELATION  OF  RAINFALL  AND  TEMPERATURE  TO  GROWTH. 

Whatever  is  said  here  about  the  behavior  of  the  guayule  in  regard 
to  growth-rhythm  must  be  understood  to  apply  to  the  region  of  North 
Zacatecas,  where  the  data  which  appear  beyond  in  detail  were  obtained. 
It  is  believed,  however,  that  the  generalizations  are  approximately  true 
for  the  whole  area  of  distribution.4 

The  grand  period  of  growth  falls  in  the  warm  season,  when  super- 
ficial soil-water  is  normally  most  abundant  and  when  the  night  as  well 
as  the  day  temperatures  are  most  effective.  If  the  rainfall  is  subnormal, 
the  drought  so  caused  at  this  time  results  in  very  slow  growth,  made 
possible  only  by  the  meager  amount  of  water  that  reaches  the  plant 
from  the  subsoil,  derived  in  part  from  the  earlier  and  usually  small  rainfall 
of  the  previous  winter,  together  with  the  more  immediately  available 
supply  from  insufficient  rains.  This  is  only  another  way  of  saying  that, 
in  the  region  above  described,  water,  as  compared  with  the  otherwise 
usually  favorable  conditions,  is  the  prime  condition  for  growth,  and  we 
may  best  see  what  the  habits  of  the  plant  are  by  observing  what  growth 
takes  place  in  relation  to  the  rainfall.  The  extreme  possibilities  would 
be  expected  to  be  shown  by  plants  under  irrigation  during  every  season. 
The  observed  growth  in  such  plants,  even  in  the  presence  of  abundance 
of  soil-moisture  during  November,  December,  and  January,  is  exceedingly 
small  in  amount.  Had  the  soil-moisture  been  reduced,  say  in  Septem- 
ber, so  as  to  bring  on  a  period  of  dormancy  in  the  plant  during  October 
and  November,  it  may  well  be  believed  that  a  much  more  marked  growth 
might  have  occurred  during  the  period  following,  when  in  point  of  fact 
little  growth  actually  occurred.  This  behavior  would  be  in  accord  with 
our  general  knowledge  of  growth -rhythm. 

Although  I  have  made  no  observation  of  positive  value  in  this 
regard,  it  is  said  by  supposedly  competent  observers  that  the  guayule 
in  the  field  may  be  expected  to  flower  at  any  time,  and  that  it  has  been 
seen  to  do  so  in  every  month  of  the  year.  Flowering,  however,  usually 
involves  some  foliage-stem  growth  as  well;  and  so  the  evidence  favors, 
or  at  any  rate  does  not  contradict,  the  view  that  growth  may  ensue  at 
any  time  of  the  year.  Because  of  the  unfavorable  night  air-tempera- 

1  Cf.  Livingston,  1906.  3  Escobar,  1910. 

2  Tolman,  see  Spalding,  1909.  *  Cf.  Bray,  1906. 


The  Environment.  21 

tures  of  the  cooler  period,  however,  the  total  amount  of  growth  will  not 
be  great,  to  which  result  the  less  effective  growing  day  temperatures 
contribute.  The  evidence  shows  further  that  growth  in  January,  e.g., 
will  ensue  upon  a  period  of  rest  coupled  with  an  unusually  favorable 
rainfall,  spread  over  time  enough  to  produce  a  marked  rise  in  the  avail- 
able soil-moisture  as  far  down  as  the  shallow  roots.  The  times  at  which 
this  conjunction  of  conditions  may  occur  is  indicated,  negatively  at 
least,  when  it  is  said  that  no  growth  in  field  plants  was  observed  till 
May  in  spite  of  the  rain,  as  indicated  in  table  i  and  the  accompanying 
diagram  (fig.  3). 

Not  only,  indeed,  did  no  growth  occur,  but  the  guayule  plants  in 
the  field  in  widely  separated  localities  showed  a  marked  need  of  water, 
a  condition  still  more  evident  in  April  1909.  On  the  nth  of  Novem- 
ber, at  Jaguey,  10  miles  northeast  from  Cedros,  the  leaves  were  in  a 
very  much  shriveled  condition.  Leaf -fall  began  toward  the  middle  of 
December,  the  upper  leaves,  which  are  not  cast  off,  being  at  this  time 
in  a  distinctly  flaccid  condition.  At  this  time  the  irrigated  plants  showed 
signs  of  leaf-fall,  but  for  some  time  only  the  lowermost  on  the  season's 
growth  of  stem  were  involved,  while  in  the  field  plants  all  the  fully  de- 
veloped leaves  fell  away  at  the  same  time. 

Although,  as  above,  seen  it  appears  probable  that  growth  may 
take  place  under  favorable  moisture  conditions  even  in  the  winter, 
there  is  little  evidence  (Chapter  III)  that  the  amount  is  ever  anything 
but  small.  The  internodes  are  short,  and  thus  is  produced  a  crowding 
of  the  leaves,  which  by  summer  growth  would  be  spread  apart,  and  the 
structural  marks  between  the  two  grand  periods  of  growth  are  less  ob- 
vious. As  will  be  seen  later,  the  dependence  which  may  be  placed  in 
these  marks  as  indicating  the  age  of  the  plant  is  not  materially  disturbed 
by  this  circumstance. 

RELATIVE   HUMIDITY. 

Unfortunately  no  instruments  were  available  at  Cedros  for  the 
study  of  relative  humidity,  and  it  is  especially  regretted  that  an  atmom- 
eter  after  Livingston's  design  was  not  at  hand.  The  only  data  obtainable, 
aside  from  my  general  observations,  are  those  issued  from  the  Observa- 
torio  de  la  Bufa  at  Zacatecas.  A  curve  of  tentative  value  based  on  these 
is  presented  in  fig.  3,  and,  while  this  can  be  regarded  as  only  approxi- 
mate, it  serves  to  indicate  that  the  relative  humidity  is  relatively  high 
at  Cedros  (though  not  as  high  as  at  Zacatecas) ,  and  that  there  is  a  some- 
what prolonged  summer  period  of  high  humidity.  The  following  re- 
marks accord  in  general  with  these  conclusions. 

Dew  is  frequent  during  the  cooler  months,  and  was  sufficient  to  run 
off  the  roof  of  the  house  occupied  as  a  laboratory,  the  material  being  of 
painted  canvas.  The  dew-point  is  always  approached  closely  at  night 
and  usually  passed  in  winter  and  during  the  rainy  summer  season.  The 
high  relative  humidities  which  occur  at  all  times  during  the  night,  and 
in  certain  situations  during  the  day,  at  least  during  growing  periods, 
are  reflected  in  the  vegetation.  Only  when  this  factor  is  taken  into 
consideration  can  we  explain  the  pronounced  contrast  seen  between 


22 


Guayule. 


the  vegetations  of  the  north  and  south  facing  slopes  (Lloyd,  1909),  and 
the  peculiar  distribution  of  certain  plants,  notably  epiphytic  species. 
A  most  instructive  example  is  offered  by  Tillandsia  ciliata,  which  is  to  be 
found  epiphytic  chiefly  on  the  ocotillo  (Fouquieria  splendens),  on  slopes, 
mostly  steep,  where  the  drainage  of  cool  air  of  high  relative  humidity 
passes  downward  from  higher  levels.  The  ocotillo  itself  grows  in  the 
more  arid  soil  of  southerly  slopes.  The  Tillandsia  ("pastle")  occurs  on 
other  shrubs  also  wherever  the  most  favorable  humidity  conditions  are 
to  be  found,  namely ,  in  arroyos  and  narrow  canadas  receiving  air-drainage 
from  adjacent  high  land,  and  I  have  seen  a  small  amount  in  open  flats 
many  miles  from  the  mountains,  where,  during  the  rainy  season,  water 
stands  for  some  time  over  large  areas,1  thus  producing  similar  conditions 
in  less  marked  degree. 


\ 


\/ 


JAI*  FEB..          MAR.  APR.  MAY  JUNE          JULY  AUG.  SEP^  OCT<  NOV.          OEU 

FIG.  3. — Monthly  precipitation  at  Cedros,  and  relative  humidity  at  Zacatecas  city. 

We  may  therefore  conclude  that  the  atmospheric  humidity  in  this 
region  is  for  a  desert  markedly  favorable  for  vegetation,  and  may  be 
called  into  account  to  explain  the  denser  total  growth  of  this  desert  as 
compared  with  the  region  immediately  about  Tucson,  Arizona.  What 
biological  relations  between  plant  structure  and  the  conditions  described 
above  may  be  found  is  a  problem  for  the  future,  the  importance  of  which 
I  have  elsewhere  pointed  out  (Lloyd,  19086).  Ross  (1908)  refers  to  the 
occurrence  of  dews  in  the  guayule  region  and  suggests  that  the  dense  tri- 
chome  structure  may  be  related  to  the  absorption  of  atmospheric  mois- 
ture, but  offers  no  evidence.  At  the  present  time  we  may  do  little  more 


1  As  in  the  "laguna"  in  the  Camacho  bolson,  east  from  that  place. 


The  Environment.  23 

than  attribute  to  the  high  vapor-tension  a  general  dampering  effect  upon 
evaporation,  both  from  the  plant  and  from  the  soil,  but  it  is  not  improb- 
able that  research  will  discover  plant-structures  which  are  specifically 
related  to  atmospheric  humidity,  especially  as  it  has  been  shown  (Lloyd, 
19050)  that  the  ocotillo  and  probably  other  plants  have  the  ability  to 
take  advantage  of  rain  which  has  not  yet  reached  the  earth. 

TOPOGRAPHY  AND  SOIL. 

The  surface  of  the  high  plateau  of  Mexico  on  which  the  guayule 
finds  its  home  is  broken  up  into  mountain  ranges  of  various  extent,  sep- 
arated by  wide,  flat  valleys  or  "bolsones."  The  middle  reaches  (playas) 
of  these  valleys  are  nearly  level  and  have  a  deep,  fine,  alluvial  soil, 
containing  a  vast  amount  of  capillary  water.  In  this  soil  the  mesquite 
is  generally  found  in  abundance,  and  often  of  large  size.  Within  these 
flats  are  frequently  found  more  or  less  extensive  areas  (alkali  spots,  salt 
spots)  where  salts  have  accumulated  and  where  the  salt-bushes  (Atriplex 
'sp.)  only  may  be  found. 

From  the  periphery  of  these  alluvial  plains,  extending  to  the  foot- 
hills of  the  mountain  ridges,  is  a  gentle  slope  of  low  gradient,  the  foot- 
slope,  characterized  by  a  gravelly  soil  (plate  5),  which  becomes  more 
and  more  stony  as  the  foot-hills  are  approached.  Here  the  soil  is  fre- 
quently very  shallow  and  may  be  confined  to  the  crevices  of  the  under- 
lying rock.  This  condition  becomes  still  more  marked  in  the  hills  proper, 
where  the  edges  of  the  strata  are  often  exposed  and  where  the  vegetation  is 
confined  to  the  intervening  fissures.  The  most  widely  distributed  plants 
of  the  foot-slope  and  adjacent  ridges,  and  therefore  the  most  characteristic, 
are  the  alvarda  or  ocotillo  (Fouquieria  splendens) ,  the  palma  samandoca 
(Samuelld  earner osa  Trelease),  and  the  Cedros  sotol  (Dasylirion  cedro- 
sanum  Trelease)  -1  The  gobernadora  or  Mexican  grease  wood  (Covillea  sp.) 
is  also  a  very  common  plant  of  the  foot-slopes  and  ridges,  but  is  to  be 
found  also  in  the  alluvial  plains  and  is  therefore  less  characteristic. 

Of  the  species  of  Parthenium  found  in  the  region,  the  guayule  is 
confined  to  the  foot-slopes  and  foot-hills,2  being  also  abundant  in  hills 
not  above  about  7,000  feet  in  altitude.  It  is  therefore,  like  some  of  its 

1  Dasylirion  cedrosanum  Trelease  (n.  sp.). 

Subacaulescent.  Leaves  slightly  roughened  on  the  dorsal  angles,  pale,  the 
upper  face  glaucous,  somewhat  fibrous-brushy  at  tip,  broad  (20  mm.),  1.5  m.  or 
more  long:  prickles  mostly  10  to  15  mm.  apart,  yellow  or  at  length  reddened  at 
tip,  3  to  5  mm.  long,  moderately  heavy,  upcurved  or  hooked,  the  whitish-yellow 
intervening  margin  roughened  by  minute  hyaline  tipped  denticles.  Branches  of 
the  narrow  inflorescence  rather  elongated,  about  7  by  60  mm.  Fruit  narrowly 
elliptical,  4  to  5  by  7  to  9  mm.,  deeply  and  acutely  notched,  the  style  much  shorter 
than  the  wings. 

Cedros,  Zac.,  Mexico,  Lloyd,  No.  118 — the  type,  No.  82,  1908;  Kirkwood, 
No.  96,  1908. 

Allied  to  D.  wheekri  and  D.  graminifolium,  from  both  of  which  it  differs  in 
its  smaller  fruit  not  widened  upwardly  and  with  shorter  style  more  conspicuously 
surpassed  by  the  wings  A(fig.  4,  and  on  the  extreme  right  of  fig.  A.,  pi.  i). 

The  type  is  in  the  Herbarium  of  the  Missouri  Botanical  Garden;  cotypes  in 
the  Gray  and  National  herbaria. 

2  It  is  generally  believed  by  those  familiar  with  the  plant  that  it  affects  more 
particularly  the  south  slopes,  and  this  accords  in  general  with  my  observations, 
though  it  must  not  be  inferred  that  it  does  not  grow  at  all  on  north  slopes. 


24 


Guayule. 


associates  above  mentioned,  an  "edaphic"  species,  found  only  where 
the  ground  is  stony.  In  the  alluvial  plains  one  meets  only  an  occasional 
isolated  plant,  but  if  the  plain  is  traversed  by  a  low  ridge  of  gravelly 
ground,  even  if  the  surface  is  raised  only  a  few  inches  above  the  surround- 
ing area,  the  guayule  may  be  found.  In  the  fine  soil  of  the  plain,  on  the 
other  hand,  the  mariola  (Prathenium  incanum  H.  B.  K.)  and  the  annual 
species  P.  hysteropharus  grow  in  abundance,  though  the  mariola  is  com- 
monly associated  with  guayule  on  the  foot-slopes  and  hills.  This  asso- 
ciation of  guayule  and  mariola  frequently  misleads  the  inexperienced 
observer  in  estimating  the  amount  of  guayule  which  may  be  found  in  a 
given  area. 

Why  the  guayule  does  not  grow  in  the  fine  alluvium  is  not  clear, 
and  is  a  question  often  asked  by  persons  familiar  with  the  facts.  Any 
reasons,  aside  from  those  mentioned  above,  which  may  be  assigned  are 
at  present  of  only  speculative  value,  but  some  reference  may  properly 
be  made  to  them. 


FIG.  4. — Dasylirion  cedrosanum  T  release.     Type  material  in  the  lower  row.     Above,  for  comparison 
fruits  of  D.  wheeleri  at  the  left  and  of  D.  graminifolium  at  the  right.      X  3/1. 

Guayule  is  confined  practically  to  the  Cretaceous  region  of  the  Cen- 
tral Plateau,  and  therefore  to  highly  calcareous  soil  (see  Chapter  IX). 
It  may  very  well  be  that  the  plant  is  sensitive  to  even  a  slight  acidity, 
and  therefore  prefers  a  soil  with  a  very  small  amount  of  humus.  Certain 
experimental  results  referred  to  beyond,  while  not  conclusive,  indicate  that 
this  explanation  may  apply  during  the  period  of  germination,  but  it  has 
been  found  that  the  absence  of  lime  is  not  a  hindrance  to  maturer  plants. 

It  is  a  popular  notion  that  the  plant  "rots"  in  situations  where 
water  is  relatively  abundant,  and  that  for  this  reason  it  is  not  to  be  found 
in  "bajillos"  or  low  places.  It  is  true  that  for  a  considerable  period  in 
the  summer  season  practically  mesophytic  conditions  prevail  in  many 
areas  within  the  flats,  especially  in  the  frequent  slight  depressions.  Here 
annual  weeds  grow  in  profusion,  and  a  number  of  species  of  desert  shrubs 
flourish.  Among  these  is  the  mariola,  the  seeds  of  which  germinate 
freely  among  the  dense  vegetation  of  shrub  and  weed,  and  in  one  season 


PLATE  5 


A.  Station  2,  Quadrats  5  and  6,  foot-slope  of  Sierra  Zuluaga. 

B.  Station  3,  Quadrat  !,  near  Cedros.     A  good  stand  of  mature  plants. 


LLOYD 


PLATE  6 


Plants  from  Quadrats  5  and  6.  Station  2. 


The  Environment.  25 

attain  a  height  of  a  foot  or  two ;  but  this  is  not  true  of  the  guayule.  That, 
however,  the  mere  quantity  of  water  or  the  density  of  the  vegetation 
are  not  the  determining  factors  is  shown  by  experimental  evidence, 
while  in  the  field  are  to  be  found  numerous  instances  of  plants  which 
have  germinated  in  the  dense  shade  and  dampness  found  beneath  the 
dead  leaves  of  the  sotol  and  in  crowded  conditions  produced  by  other 
plants,  such  as  the  lechuguilla.  Indeed,  these  are  frequently  the  only 
conditions  under  which  the  plant  gains  a  foothold.  It  therefore  does 
not  appear  probable  that  the  abundance  of  water  or  the  density  of  the 
vegetation  is  the  determining  factor  in  preventing  the  guayule  from  get- 
ting a  start;  hence  we  may  infer  that  the  conditions  below  the  surface 
must  be  understood  before  an  explanation  may  be  had.  The  edaphic 
habitat  of  the  plant  suggests  that  the  mechanical  conditions  of  the  allu- 
vial soil  are  unfavorable,  owing  to  meager  aeration,  in  connection  with 
which  the  humus  conditions  also  may  have  to  be  taken  into  account. 

DENSITY  OF  GROWTH. 

Of  great  importance  economically  as  well  as  to  the  student  of  vege- 
tational  problems  is  the  number  of  plants  per  unit  of  area,  both  abso- 
lute and  relative.  The  operations  of  the  forester  rest  upon  this  datum 
in  the  first  instance,  as  this,  together  with  the  size  of  the  individuals, 
forms  the  basis  of  calculations  of  the  available  tonnage  per  acre.  It 
will  readily  be  understood  that  any  estimate  on  a  large  scale  will  involve 
a  necessarily  large  error,  since  it  would  be  impossible  to  do  more  than 
proceed  on  the  basis  of  sample  counts  combined  with  acreage  and  esti- 
mates of  size.  This  can  frequently  be  done  with  great  accuracy  by  persons 
who  have  had  practical  experience  in  taking  guayule  from  the  field, 
especially  if  the  judgment  be  checked  by  survey  and  sample  counting 
and  weighing.  The  following  tables,  the  data  for  which  were  obtained 
by  accurate  measurement,  will,  however,  serve  a  useful  purpose  in  indi- 
cating a  method  of  making  estimates,  as  well  as  in  furnishing  indications 
of  actual  conditions.  For  the  purpose,  quadrats  of  100  square  meters 
were  laid  out  by  means  of  a  steel  tape,  the  data  obtained  attaching  to 
the  guayule  plants  within  each  such  quadrat. 

The  weights  in  the  following  tables  are  field  weights.  For  dry 
weights  a  reduction  of  20  to  25  per  cent  is  necessary.  As  field  weight  is 
usually  assumed,  however,  I  have  followed  the  usage  and  have  not  applied 
the  above  correction. 


26 


Guayule. 


TABLE  4. 

Two  adjoining  quadrats,  each  of  100  sq.  meters,  on  a  loma  or  ridge  extending 
toward  the  Sierra  Zuluago,  about  10  miles  north  of  Cedros  (plate  5,  fig.  A).  All  the 
plants  were  pulled  up  and  sorted,  each  package  containing  plants  of  similar  size  and 
habit.  The  packages  were  then  grouped  into  classes  arbitrarily,  and  a  typical  plant 
for  each  class  photographed  (plate  6) .  The  age  of  this  plant  was  carefully  estimated 
and  checked  by  estimating  the  ages  of  a  number  of  similar  plants.  (March  29,  1908.) 


Class. 

No.  of 

plants  in 
package. 

Weight  of 
package. 

Average 
weight  of 
individuals. 

Estimated 
age. 

Average 
height  of 
class. 

Ibs. 

Ibs. 

years. 

cm 

2O 

15 

0-75  ] 

I  

20 
20 

17 
22.5 

0.85 

I.  12    f 

15  to  18 

58 

20 

17 

o.85J 

20 

13 

0.65 

20 

10.5 

0.52 

II 

20 
20 

10.5 

0-45 
0.52 

ii  to  13 

30 

20 

9-5 

o.47 

2O                12 

0.6 

f    50    !     19 

0.38] 

in  ;  

40 

4° 

10.25 
"•5 

0.25  1 
0.29  [ 

7  to  10 

25 

I     40 

10 

0.25  J 

[60           f  8.25 

0.14  ] 

IV  

1     120 
50 

18 

7-5 

o.iS  I 
0.14  ( 

5  to  7 

20 

I       70 

ii 

0.16  j 

'       60 

7 

0.  II 

80 

8 

0.  I 

60 

6 

0.  I 

v  

TOO 
I83 

7-75 
18.25 

0.08 

O.I 

i  to  5 

15 

100 

"-5 

0.  12 

100 

10 

0.  I 

18 

2 

0.  II 

Total  J 

1371 

303.0 

1  685  plants  in  100  square  meters. 


Class  V  is  made  up  of  all  sizes  up  to  the  maximum  indicated.  In  plate 
6,  figs.  5  to  7,  examples  of  three  sizes  and  ages  are  shown.  The  weights 
and  estimated  ages  of  all  the  plants  in  plate  6  are  shown  in  table  5. 

TABLE  5. 


Class. 

Age. 

Height. 

Weight. 

I.. 

years. 
I  c 

cm. 

18   <; 

II.  . 

1  1 

\0 

6 

Ill  

8 

2  'I 

i  87? 

IV  
V.. 
VI  .. 
VII  

7 
5 

2 

I 

20 
IS 

0.875 
0.05 

The  Environment.  27 

TABLE  6. — Station  8,  quadrat  2  (too  square  meters  on  low  ridge  just  north  of  Cedros). 


Class. 

No.  of 
plants  in 
class. 

Total  weight. 

Individual 
weight. 

Height. 

Age. 

Ibs.     oz. 

Ibs.    oz.                         cm. 

years. 

i 

3      14 

3      *4                         70 

0) 

I 

i 

3      1° 

3       10                         60 

20 

i 

3      10 

3      10                        70 

22 

i 

3        6 

36                       60 

15  to  18 

3 

10        8 

38                  60  to  68 

20 

4 

i 

9      14 
2        4 

2        7-5            45  to  55 
24                      68 

16  to  18 
20  (about) 

II.. 

i 
I 

2        4 

2           8 

2       4                     55 
28                      50 

20 

15  to  18 

I 

2         10 

2         IO                                  50 

15  to  18 

i 

2         10 

2         TO 

70 

(0 

4 

8      14 

2           3-5 

45  to  50 

2l8t020 

4 

6        8 

I         10 

40  to  45 

15 

i 

i        4 

i        4 

46 

15 

i 

i      14 

i      14 

65 

17  to  18 

Ill  

4 
I 

6      14 

I        00 

i      11.5 

I        00 

45  to  50 

40 

IS 

7  or  8 

I 

i        3 

1        3 

37 

7  or  9 

I 

i        6 

i        6 

40 

10  tO  12 

I 

i        7 

i        7 

43 

12 

I 

I        OO 

I        OO 

35 

9 

2 

I     14 

•  •    15 

40 

7  to  8 

2 

I     14 

•  •    15 

35 

2  10  to  15 

IV  

I 

II 

II 

?? 

10 

I 

II 

II                     40 

8 

2 

i        6 

..    II                38 

Ji5 

2 

i        4 

.   .        10                                 40 

x 

8 

-.8                     35 

*9  to  10 

I 

6 

6 

3° 

9 

I 

4 

4 

3° 

8 

I 

3 

3 

3° 

10 

I 

2.75 

2.375 

27 

6  to  7 

I 

2.125 

2.125 

23 

5 

V  

I 

2     12  C 

2125;           27 

6  to  7 

I 

*  .  *  *  0 
1.5 

*•  •  *     0     i               *  / 
1-5                           20 

(') 

1 

2 

3-75 

1.875     |                27 

6  to  7 

I 

1.125 

I  .  125                     20 

5 

2 

.875 

i-5                 13 

3  to  4 

14 

(*) 

i 

34 

(5) 

Total  

75 

"83       14 



.... 

1  Old  scraggly  plants  whose  age  it  was  impossible  to  determine  even  approximately. 

2  Slowly  growing,  densely  branched  plants. 
»  Tall  and  slender. 

<  Retoflos. 

*  Very  small. 

•  Classes  I-IV,  inclusive. 


28  Guayule. 

TABLE  7. — Station  8,  quadrat  i  (100  sqtiare  meters  on  low  ridge  just  north  of  Cedros, 
July  20.  1908). 


Class. 

No.  of 
plants  in 
package. 

Total  weight. 

Individual 
weight. 

Height. 

Ibs.          oz. 

Ibs.          oz. 

cm. 

i 

6            4 

6             4 

70 

i 

6            4 

64 

I                             

i 

4            8 

4             8 

'  ' 

i 

4             2 
4             6 

4             2 
4            6 

01070 

i 

4            3 

46 

i 

4             2 

4             2             60  to  70 

i 

3           M 

3           U 

i 

3           I0 

3          10                 60 

II....                 

i 

3          10 

3          10 

i 

3             4 

3            4            40  to  45 

i 

3          0° 

3          oo            50  to  60 

i 

2               I4 

14 

i 

2               12 

12        !                  55 

i 

2               12 

12 

i 

2               12 

12 

Ill  

i 

2                  8 

8 

2 

4            8 

4 

I 

2               00 

00 

I 

IO              00 

oo 

5 

10              00 

00 

i 

I               00 

00 

10 

IO                  2 

0.2 

10 

10               12 

I 

IV 

10 

II               12 

3 

45 

I 

I                  6 

6 

45 

I 

I               10 

IO 

I 

I               12 

12 

6 

10               12 

13 

45  to  60 

f     4 

3          oo 

12 

35 

V  

21 

8 

.  .  " 

3ot035 

5 

I               00 

3 

3° 

I     5 

4 

0.8 

15  to  20 

Total  

81 

135            5 

1  Retonos. 


The  Environment. 


TABLE  8. — Station  g,  quadrat  i  (100  square  meters,  on  a  2$-degree  northeast  slope, 
in  the  hills  east  of  Cedros) . 


Class. 

No.  of 

plants  in       Total  weight, 
package. 

Individual 
weight. 

Height. 

Ibs.       oz. 

Ibs.       oz. 

cm. 

I  

f     I               3       12 

3      12 

60  to  65 

3        8 

38                         60 

2 

4      10 

5                 50  to  60 

II  .... 

2 

2 

4        4 
4        6 

2                      55 
3                  35*040 

5 

II           O 

3                        60 

1 

2          O 

o                       45 

5 

9        6 

14                   40  to  45 

3 

5        8 

13 

5° 

i  . 

I         10 

10                        50 

III...                . 

2 

3        ° 

8                      60 

5 

6        12 

5.6                  40 

i 

i        4 

4 

45 

2 

2        4 

2 

45 

1 

I          O 

o                        40 

4 

3      12 

••15                       35 

i 

IO 

10 

45 

4 

2            2 

.-        8.5 

25  to  3  5 

i 

.  .'       .  . 

5 

i 

••        3-5 

i 

2.125 

_ 

2 

••            3.25 

i  .625 

I 

1.5 

I 

1-375 

IV 

I 

1.25 

I 
I 

i  .  18 
.875 

I 

•  75 

2 

1.25 

.625 

I 

•  5 

20 
20 

5 
i-375 

ill 

•  • 

2O 

1.31   i    

4 

.125!    ..          .03 

8 

.125 

.015 

14 

Total  

133 

263      

1  Retonos  of  small  size. 

2  Total  weight,  classes  I  to  III.  inclusive. 


30 


Guayule. 


TABLE  9. — Station  2,  quadrat  7,  April  3,  1909. 

[The  data  for  this  table  were  obtained  by  pulling  up,  sorting,  and  weighing  all  the 
plants  on  100  square  meters  in  a  guayule  field  from  which  all  the  plants  above 
40  cm.  tall  had  already  been  taken.  (Plate  i,  fig.  A.)] 


No.  of  plants. 

Weight. 

Average  weight. 

Height. 

Ibs.     oz. 

oz. 

cm 

100 

25        8 

4 

25*035 

60 

ii 

3 

3° 

60 

12 

3-2 

25*053 

60 

10        8 

2.8 

20  to  3  5 

31 

7        4 

3-7 

25  to  30 

60 

13      •  • 

3-5 

22  tO  30 

5° 

4        8 

1-4 

20  tO  30 

5° 

7        4 

2.32 

20  to  25 

5° 

6        12 

2 

20  to  25 

50 

4        4 

1.26 

20  to  25 

5° 

5        8 

1.76 

20  tO  25 

64 

3        4 

0.8 

18  to  20 

61 

2         .  . 

°-5 

15 

'9 

0.125 

7  c  c 

755 

112           12.12^ 

All  seedlings  of  1908,  except  one  of  1907. 


TABLE  10. — Station  9,  quadrat  2,  April  14,  1909  (100  square  meters,  ridge  of  lorna  in 
hills  (El  Potrero),  east  of  Cedros). 


No.  of  plants. 

Weight. 

Average  weight. 

Height. 

Remarks. 

Ibs.         oz. 

oz. 

cm. 

4          4 

68 

70 

3 

48 

65 

3 

48 

65 

2              8 

40 

63 

Scrubby. 

3        I2 

60 

62 

I            12 

28 

60 

2              4 

36 

60 

2 

32 

57 

Rather  scrubby. 

I            12 

28 

55 

Do. 

2 

36 

53 

5        12 

92 

5° 

Spread  100  cm. 

2              4 

36 

5° 

2            12 

22 

44 

3        12 

15 

40 

3-5 
i 
2.5 

3-5 
•5 

24 
24 
23 

4  to  5  years  seedling. 
Badly  developed. 
Seedling,  5  years. 

2.5 
0.5 
-.          0.5 

•5 
•5 
•5 

16 
ii 
14 

3  years  retofio. 
Seedling,  2  years. 
Seedling,  3  years. 

24 

41          6 

The  Environment. 


TABLE  u. — Station  10,  April  5,  1909  (quadrat  of  100  square  meters). 
[On  a  southerly  slope  10  kilometers  north  of  the  Cerritos  de  los  Calzones.] 


No.  of  plants. 

Weight. 

Average  weight. 

Height. 

Remarks. 

Ibs.        oz. 

oz. 

cm. 

I 

2           4 

36 

65 

NOTE.  —  The  shrub  of 

3 

8 
4          4 

8 
23 

5° 

40  to  50 

this     region     is     rough 
looking  and  rather  badly 

5 

6 

I9 

40  to  50 

attacked  by  insects.     A 

5 

9 

29 

45 

good    deal    of    witches' 

6 
4 

6            12 

5          8 

18 

22 

35*050 
301050 

broom,  and  many  plants 
attacked  at  the  base  by 

8 

6          4 

12-5 

35  to  40 

borers. 

8 

6 

12 

35  to  40 

2 

8 

4 

30  to  40 

7 

6          8 

14.8 

30  to  40 

5 

5        12 

18 

35 

7 

5          8 

12.6 

35 

10 

6 

9.6 

35 

8 

6          8 

13 

35 

5 

3          8 

II  .2 

30  to  35 

16 

4 

4 

3°  to  35 

5 

4 

13 

25to35 

10 

3 

4-8 

20  to  40 

10 

2               8 

4-4 

25  to  30 

16 

2            12 

2-75 

25 

12 

'4         .- 

5-3 

20  to  30 

6 

4 

0.6 

15  to  20 

7 

Seedlings,  2  to  3  years  old. 

i 

Seedlings,  2  years  old. 

7 

Seedlings,  about  3  yrs.  old 

8 

Retonos. 

3 

Small  plants. 

1  86 

101           4 

1  Scant. 
TABLE  12. — Station  n,  near  Caopas,  April  6,  1909. 


No.  of  plants. 

Weight. 

Average  weight. 

Height. 

Ibs.     oz. 

01. 

cm. 

I 

2          6 

38 

50 

I 

I        IO 

26 

5° 

5 

5        8 

17.6 

40  to  50 

5 

7        4 

22 

35  to  40 

6 

6       o 

16 

35  to  40 

i 

i      .... 

16 

35 

8 

6      

12 

30  to  40 

5 

5      

16 

30  to  40 

7 

5      

ii.  4 

30  to  40 

8 

4      .... 

8 

3°  to  35 

9 

2          8 

4-4 

3°  to  35 

8 

2          8 

5 

3«> 

12 

4      12 

6-3 

3° 

15 

2         .... 

2  .  I 

25 

21 

3      

2-3 

20  to  30 

12 

2           8 

3-3 

20  tO  25 

37 

3        8 

1-5 

20  tO  25 

10 

2           8 

4 

20  to  25 

25 

2          4 

i  .4                       20 

3° 

i       .... 

0.5                      20 

i? 

10 

0.6                 1  5  to  20 

21 

4 

0.2                                  15 

3 

2.5 

0.8 

15 

12 

10 

0.8                101015 

279 

'?i      14-5 



1  7.100  pounds  per  hectare. 


Guayule. 


TABLE  13. — Station  12,  foot-slope  ridges  south  from  Apizolaya, 
April  10,  1909.     (Plate  7,  and  plate  i,fig.  B.) 


No.  of  plants. 

Weight. 

Average  weight. 

Height. 

Ibs.        02. 

OS. 

cm. 

2 

4           8 

35 

60  to  70 

5 

10            .... 

32 

60  to  70 

5 

10            

32 

60  to  70 

5 

9          8 

30-4 

60  to  70 

5 

9          8 

3°-4 

60  to  70 

5 

9        

28.8 

60  to  70 

5 

9              

28.8 

60  to  70 

5 

g 

25.6 

60  to  70 

5 

7        12 

24.8 

60  to  70 

5 

7        

22.4 

60  to  70 

5 

6          8 

20.8 

60  to  70 

i 

3        

48 

60 

5 

9          8 

30.4 

60 

9 

6        

10.7 

60 

5 

2              8 

8 

50 

7 

3        

7 

5° 

9 

4         

7-i 

5° 

7 

38|8 

So 

5 

4          4                      13-6 

40  to  50 

6 

4          8 

12 

45 

5 

4         

12.8 

40 

5 

3          8 

II  .2 

40 

5 

4          4 

I3.6 

40  (i  at  50) 

10 

3         

4-8 

40 

6 

5          4 

14 

35t045 

10 

2           4 

3-6 

35  to  40 

12 

2  e 

2              4 
2 

3 

I       1 

30 

3O 
w 

.> 

20 

3          4 

O 

2.6 

30 

25 

2               8 

1.6 

3° 

28 

112                                 I 

25  to  30 

20 

2            12                                 2.2 

25*030 

, 

10                                5 

3° 

13 

6.5 

i8t023 

4 

2.75 

18  to  23 

5 

3.5 

* 

22 

Add,  making  additional  weight  — 

10 

Scraps.  .  . 

3          4 
4         

40  to  60  (scrappy) 

3" 

172         5 

Summarizing  the  above  results,  and  including  data  from  other  sources, 
we  have  as  follows: 


Station. 

Quadrat. 

No.  of  plants. 

Station. 

Quadrat. 

No.  of  plants. 

4 

I 

5° 

8 

I 

81 

5 

I 

275 

9 

I 

13° 

2 

3 

360 

2 

7 

755 

2 

4 

270 

9 

2 

24 

2 

i 

285 

10 

186 

3 

i 

3° 

1  1 

279 

2 

1  5  and  6 

685 

12 

3" 

8 

2 

75 

'Averaged. 


A.  Quadrats  (station  12)  in  a  very  dense  growth.     Apizolaya. 

B.  The  same,  the  guayule  removed. 


The  Environment. 


Here  there  is  a  range  in  numbers  of  plants  from  2,400  to  75,500 
plants  per  hectare,  but  the  meaning  of  these  figures  can  not  be  under- 
stood unless  the  size  of  the  plants  is  taken  into  consideration.  From 
the  point  of  view  of  business  opportunism,  a  stand  of  2,400  plants  per 
hectare  may  be  better  than  one  of  much  higher  figures,  while  for  one  who 
is  looking  for  a  basis  for  permanent  investment  other  questions  of  rela- 
tive sizes  and  numbers  of  plants  arise,  the  answer  to  which  involves  an 
explanation  of  the  rate  of  reproduction  in  the  field.  This  subject  will 
be  treated  in  detail  in  Chapter  IV,  it  being  our  purpose  here  to  show 
the  actual  condition  as  viewed  by  one  who  is  estimating  the  tonnage  per 
unit  of  area. 

If  we  refer  back  to  table  4,  we  will  observe  that  the  two  quadrats 
contained  1,371  plants,  the  average  weight  of  which  was  a  little  over  3.5 
ounces.  Of  these,  however,  only  80  were  large  enough  to  be  gathered, 
namely,  those  about  i  pound  or  over  in  weight;  though  if  the  land  were 
being  exploited  smaller  ones  would  be  taken,  say  those  weighing  above 
half  a  pound.  This  would  include  all  of  the  plants  in  classes  i  and  n, 
weighing  in  the  aggregate  about  58.75  pounds,  or  5,875  pounds  per  hec- 
tare, assuming  the  quadrats  to  be  fair  samples,  or  about  2.67  tons  (long). 

Treating  the  remaining  tables  similarly,  we  have  the  following 
figures: 

TABLE  14. 


Table  No. 

No.  plants 
above 
8  ounces. 

Weight  of 
these. 

Weight  per  hectare. 

No.  plants 
below 
8  ounces. 

A3fSM3£t 

plants. 

Ibs.         oz. 

Ibs. 

tens  (long). 

Ibs.      oz. 

4 

80 

58      12 

5,375 

2.4  + 

605 

o      11.75 

6 

45 

83       14 

8,39° 

3-7 

30 

I       13-8 

7 

69 

I3i         7 

i3,J40 

6     - 

12 

i       14-5 

8 

43 

69        8 

6,95° 

3-i 

90 

i        9.8 

10 

18 

40        5 

4,033 

1.8 

6 

2       5-8 

ii 

114 

83        8 

8,35° 

3-7 

98 

o      11.7 

12 

47 

43      12 

4,374 

2        — 

232 

14.9 

13 

H3 

137           0 

13,70° 

6.1 

200 

i        3-4 

It  will  need  but  a  glance  at  the  above  summary  to  show  that,  from 
the  business  point  of  view,  the  acreage  of  large  but  comparatively  few 
plants  is  the  more  valuable  to  the  purchaser  who  is  not  looking  to  the 
future,  for  the  reason  that  the  cost  of  harvesting  a  small  number  of 
large  plants  will  be  less  than  if  the  available  plants  are  large  in  number 
and  of  smaller  size,  and  because  the  larger  plants  can  be  handled  more 
readily  and  therefore  more  cheaply.  Furthermore,  it  is  much  easier 
to  determine  the  tonnage  with  fair  accuracy  where  the  plants  are  few 
and  large.  The  error  due  to  applying  data  taken  from  small  sample 
areas  to  an  extensive  area  within  which  the  sample  area  falls,  must  of 
necessity  be  large,  for  the  number  of  plants  as  well  as  their  character 
must  be  considered.  Taking  the  question  of  number  alone,  the  size  of 
the  error  on  this  score  will  be  appreciated  when  it  is  known  that  on  an 
area  of  42.7  acres  at  Station  2  (plate  i)  181  bales  of  guayule,  or  at  the 


34 


Guayule. 


rate  of  about  800  pounds  per  acre  (1,976  pounds  per  hectare) ,  were  actually 
collected.  As  this  was  gathered  under  the  rule  that  no  plants  less  than 
40  cm.  in  height  or  in  spread  were  to  be  taken,  some  plants  which  would 
run  over  8  ounces  were  doubtless  left,  but  allowing  for  this  error  probably 
riot  more  than  2,000  pounds  to  the  hectare  could  have  been  taken,  or  at 
most  i  ton  of  2,200  pounds.  On  another  area  of  30.8  acres  of  the  same 
general  character,  but  of  thinner  stand,  53  bales  or  at  the  rate  of  344 
pounds  per  acre  (about  850  pounds  per  hectare)  were  gathered. 

It  will  thus  be  seen  that  the  difficulty  in  estimating  tonnage  per 
unit  of  area  with  small  error  is  at  best  very  great,  and  this,  as  already 
said,  is  rendered  more  so  by  the  difference  in  the  character  of  the  plants. 
To  judge  of  the  truth  of  this,  one  has  but  to  examine  the  various  illus- 
trations accompanying  this  paper.  In  particular,  a  comparison  of  two 
prevalent  types  is  shown  in  plate  8,  namely,  a  slender  and  a  spreading 
type,  but  neither  of  extreme  form. 

TABLE  15. — Dimensions  of  narrow  and  spreading  types  of  shrub,  illustrated  in  plate  8. 


Narrow  type. 

Spreading  type. 

Plant. 

Weight  fresh. 

Weight  dry.       Height. 

Plant. 

Weight  fresh. 

Weight  dry. 

Height. 

Ibs.      oz. 

Ibs.      oz. 

cm. 

Ibs.      oz. 

Ibs.      oz. 

cm. 

A 

4      0 

2        II 

65 

A 

3        6 

2         4 

5° 

B 

2        0 

I            2 

48 

B 

2         12 

1      I3 

45 

C 

I        2 

•  •      T5 

46 

C 

I           O 

•  •       14 

33 

D 

..      8 

5-5 

33 

D 

..        6 

5 

23 

E 

.  .      6 

4 

28 

E 

5 

3-75 

21 

F 

•  •      3 

i.  5 

24 

F 

i-75 

0.93 

I? 

G 

..      1.25 

0.5 

23 

From  the  above  data  it  is  seen  that,  speaking  broadly,  the  weight 
of  plants  of  the  spreading  habit  is  one-third  to  one-half  greater  than  those 
of  the  narrow  type  of  similar  height,  so  that  a  stand  of  the  latter  must 
have  a  density  correspondingly  greater  to  equal  in  total  weight  a  given 
stand  of  the  spreading  type. 

As  one  looks  over  a  "field"  of  guayule,  these  apparently  minor  dif- 
ferences of  form  are  not  at  all  apparent,  because  of  the  interference  of 
other  vegetation  with  the  vision.  If  the  occasion  presents  itself  when 
more  accurate  estimates  will  be  demanded  than  at  present,  this  condi- 
tion will  have  to  be  taken  into  account.  It  should  be  further  mentioned 
that  the  weights  given  above  are  of  freshly  gathered  plants.  If  it  is 
desired  to  calculate  to  "air-dry"  shrub,  the  proper  correction  should 
be  applied,  but  as  this  is  very  variable,  according  to  the  season,  no  con- 
stant can  be  given.  It  may,  however,  be  as  great  as  22  per  cent  in  the 
dry  season. 

The  only  other  published  calculations  of  this  kind  were  made  by 
Endlich  (1905,  p.  1118),  who,  for  the  purpose  of  calculating  the  area  of 
guayule  land  necessary  to  support  the  industry,  assumes  the  average 
weight  of  the  plant  to  be  500  grams,  and  the  density  of  growth  to  be, 
by  weight,  500  to  800  kilograms  per  hectare,  or  from  1,000  to  1,600 


The  Environment.  35 

plants  per  hectare  of  500  grams  average  weight,  taking  into  account  the 
unevenness  of  distribution,  that  is,  the  more  or  less  extended  areas  where 
guayule  does  not  occur.  The  following  figures  are  deduced  from  the 
quadrats  above  detailed,  taking  all  the  plants  into  account  : 

TABLE  16. — Number  of  plants  in  given  areas. 


Table  Nos. 

No.  of 

Average  weight. 
Kilograms 

quadrats. 

hectare. 

Ounces.            Grams. 

hectare. 

4 

68,500 

3-5                 99-2 

6,795-2 

6 

7.5°° 

18.4              521.6 

3,912 

7 

8,100 

26.56            753-°             6,099 

8 

13,30° 

8.5              241.0 

3,205 

10 

2,400 

27-2              77I-I        ;      1,850.64 

ii 

18,600 

8.71            246.9       i      4,592.34 

12 

27,900 

4.12            116.79     i      3,258.72 

13 

31,100 

8.86           252.97     .      7,867.36 

Ave.  . 

22,175 

13-2             375-3 

4,672.53 

From  the  above  it  is  seen  that  the  average  in  long  tons  per  hectare 
is  4.67,  per  acre  1.85. 

The  average  weight  of  all  the  plants  on  the  quadrats  is  thus  seen 
to  be  less  than  Endlich's  estimate  by  125  grams,  or  one-fourth,  and  as 
these  sample  areas  include  the  very  best  guayule  land,  that  is,  the  densest 
areas  with  the  largest  plants  in  relation  to  the  density,  it  may  be  con-' 
eluded  that  the  present  estimate  is  more  nearly  correct.  In  estimating 
the  average  density  over  large  areas,  great  difficulties  are  met.  Endlich 
assumed  one-tenth  of  the  area  of  the  guayule  region  to  be  occupied  by 
the  shrub  at  an  average  density  of  500  to  800  kilograms  per  hectare. 
This  figure  does  not  approach  the  indications  of  our  data,  though  it 
must  be  remembered  that  these  do  not  take  into  account  poor  areas 
where  the  shrub  is  very  scattering  or  nearly  absent — as  the  Mexican 
well  expresses  it  "salteadito."  For  certain  areas,  e.g.,  one  of  1,800,000 
acres  (728,744  hectares)  which  has  been  somewhat  closely  studied  for 
the  special  purpose  of  estimating  the  amount  of  shrub  to  be  found  there, 
Endlich's  factor  was  found  to  be  very  small,  for  if  only  one-hundredth 
of  its  area  carried  guayule  in  the  quantity  of  our  general  average,  there 
would  be  as  much  as  of  one-tenth  of  it  which  carried  shrub  of  the  amount 
of  his  factor.  We  may  feel  sure,  however,  that  our  average  applies  to 
more  than  one-hundredth  of  the  total  area.  Whether  Endlich's  figure 
applies  better  to  the  total  guayule  area  of  Mexico  can  not  be  said  with 
any  certainty,  but  it  is  only  fair  to  say  that,  in  view  of  the  great  diffi- 
culties involved,  it  is  probably  as  near  the  truth  as  any  that  we  might 
venture. 


36 


Guayule. 


BIOTIC  RELATIONS. 
COMPETITION. 

The  relation  of  guayule  to  the  other  plants  with  which  it  is  com- 
monly found  associated  is  of  great  importance,  especially  if  forestry 
methods  are  contemplated.  Both  the  mutual  effect  of  each  element  in 
the  vegetation  upon  the  guayule  and  the  relative  rate  of  growth  must 
be  understood  in  order  to  judge  what  the  final  effect  in  the  struggle  for 
existence  is  likely  to  be.  To  do  this,  however,  involves  a  very  consider- 
able amount  of  sustained  observation  by  means  of  the  quadrat  method, 
first  devised  by  Clements.  Following  is  a  census  of  the  more  important 
plants  found  growing  in  association  with  the  guayule  in  quadrats  5  and 
6,  Station  2. 

TABLE   17. 


Scientific  name. 

Common  name. 

Total  No.  in  200 
square  meters. 

Parthenium  argentattim 

Guayule                          .    . 

1*71 

Agave  lechegtiilla 

Lechuguilla 

CQ 

Covillea  mexicana 

Gobernadora 

56 

Samuella  carnerosa  

Palma  samandoca  

4 

Dasylirion  cedrosanum  

Sotol  

i 

Acacia  farnesiana  

Huisache  
Sangre  de  drago 

7 
Scattered     all 

Zexmenia  brevifolia  

over. 
6 

Lophophora  williamsii  
Opuntia  megalarthra.  .               ... 

Peyote  (peyotl)  
Rastrero  

about  20 
45 

Also  the  following,  from  Station  10: 
TABLE   18. 


Scientific  name. 

Common  name. 

No.  in  100 

square  meters. 

Parthenium  argentatum  

Guayule  

1  86 

Parthenium  incanum 

Manola                           .... 

14 

Opuntia  stenopetala 

Nopal  Colorado             .  .  . 

e 

Opuntia  microdasys 

Segador 

7 

Covillea  mexicana 

Gobernadora 

8 

Opuntia  imbricata 

Gardenche 

i 

Engorda  cabra 

•j 

Sotol 

i 

Samuella  carnerosa  
Agave  asperrima  .... 

Cacti  

Palma  samandoca. 
Maguey. 

Several  small 
inconspicu- 
ous plants. 

With  few  exceptions,  these  constitute  the  dominant  vegetation  of 
the  foot-slopes  and  the  low  ridges,  though  of  course  a  number  of  other 
species  may  be  found  in  other  localities,  and  indeed  may  be  more  impor- 
tant elements  elsewhere  than  has  been  observed  to  be  the  case  in  North 
Zacatecas. 


A.  Narrow  type  of  guayule.     B.   Spreading  type  of  guayule.     (See  table  1 5.) 


The  Environment. 
A  few  of  the  more  obvious  of  these  are: 


37 


Scientific  name. 

Common  name. 

Echinocactus  palmeri  
Fouquieria  splendens  

Biznaga  burra. 
Ocotillo  or  alvarda. 
Biznaga  colorada. 
Asafran. 

Echinocactus  pringlei 

Buddleia  mamibiifolia 

The  above  enumeration  indicates  that  at  the  present  time  the  guay- 
ule  in  this  habitat  is  far  and  away  the  most  important  plant  numerically, 
and  is  therefore  dominant  in  the  usual  sense.  Whether  it  will  continue 
so — whether  its  dominance  is  waxing  or  waning — may  be  indicated  by 
the  relative  numbers  of  guayule  plants  of  different  ages  and  by  the  inter- 
action of  the  various  elements  in  the  vegetation. 

We  may  therefore  consider  briefly  each  of  the  numerically  most  im- 
portant species. 

LECHUGUILLA  (AGAVE  LECHEGUILLA). 

While  the  actual  number  of  plants  of  this  species  found  in  quadrats 
5  and  6  is  much  larger  than  that  of  any  other  save  guayule,  it  is  very 
small  compared  with  the  number  which  is  found  on  much  guayule  land 
(e.g.,  plate  5,  fig.  B). 

In  common  with  the  Agaveae,  the  plant  propagates  itself  chiefly  by 
means  of  stolons  which  lie  a  few  centimeters  below  the  surface.  In  this 
way  it  spreads  from  an  original  plant  radially,  taking  up  the  ground  as 
it  goes,  from  which  nothing  but  death  dislodges  it.  In  the  course  of  a 
few  years  it  attains  maturity,  when  a  tall  flower-stalk  is  developed;  then 
the  whole  individual,  consisting  of  a  single  cluster  of  leaves  attached 
to  a  short  (10  to  15  cm.)  and  thick  (6  to  7  cm.)  stem,  dies.  Where  the 
lechuguilla  has  occupied  the  ground  for  some  time,  it  frequently  forms 
a  dense  growth,  from  which  other  plants,  save  a  few  annuals  or  emaciated 
perennials,  are  excluded.  Its  manner  of  spreading,  by  which  it  repro- 
duces itself  vegetatively ,  enables  the  plant  to  occupy  areas  in  which  the 
soil  is  confined  to  the  crevices  of  the  rocks,  and  in  this  manner  it  may 
occupy  ground  which  is  unfit  even  for  those  desert  plants  with  which 
it  is  usually  associated.  From  it  is  extracted  the  fiber  "ixtle  tula,"  or 
"ixtle  de  lechuguilla,"  which  is  of  considerable  commercial  importance, 
and  thus  the  plant  is  of  some  value — not,  however,  sufficient  to  justify 
it  as  a  competitor  of  the  guayule.  The  method  of  vegetative  reproduc- 
tion above  noted  is  also  characteristic  of  the  guayule  (Lloyd,  igoSc), 
especially  when  growing  where  the  country  rocks  come  to  the  surface, 
but  is  in  this  plant  of  relatively  much  less  importance. 

The  mutual  behavior  of  these  two  plants  under  strong  competition 
is  not  very  easy  to  describe  precisely.  It  seems  clear  that,  with  the  excep- 
tion of  a  few  plants  which  succeed  in  gaining  a  foothold  by  germinating 
in  the  shade  between  plants  of  lechuguilla,  sometimes  being  favored 
by  the  protection  from  drying  out  and  from  cropping  by  animals  thus 
afforded,  ground  occupied  by  lechuguilla  is  much  less  favorable  for  the 


38  Guayule. 

growth  of  guayule  than  that  from  which  lechuguilla  is  absent.  For 
although  it  would  seem  that  germination  and  early  growth  are  favored 
by  the  protection  offered  by  the  lechuguilla,  as  a  matter  of  observation 
one  finds  but  few  young  plants  of  guayule  in  such  situations.  One  reason 
for  this  is,  probably,  that  the  guayule  seeds  (achenes)  find  difficulty  in 
reaching  the  soil,  because  the  leaves  of  the  lechuguilla  catch  them  and 
hold  them  in  their  axils  till  they  die,  thus  materially  reducing  the  num- 
bers which  reach  the  ground.  Aside  from  the  consideration  that  the 
lechuguilla  takes  up  from  the  soil  its  quantum  of  water,  its  effect  upon 
guayule  is  unfavorable,  therefore,  because  of  its  superior  powers  of  pro- 
gressively and  steadily  occupying  the  ground,  and  because  of  the  loss 
of  guayule  seed  by  being  caught  in  its  leaves.  Lechuguilla  appears  to 
be  an  increasingly  dominating  type  in  every  situation  where  it  gains 
a  foothold.  It  is  common  to  every  part  of  the  foot-slope  and  in  the 
hills  throughout  the  range  of  guayule.  The  great  quantity  of  it  to  be 
found  produces  in  many  parts  of  the  mesa  central  the  dominating  yellow- 
green  coloring  often  seen  there.  When  it  and  the  guayule  are  associated, 
the  green  is  dotted  by  the  gray  of  the  latter,  although  other  plants  also 
may  contribute  this  subdued  note  in  the  coloring. 

GOBERNADORA    (COVILLEA    TRIDENTATA)    AND    OCOTILLO    (FOUQUIERIA    SPLENDENS). 

These  may  be  considered  together.  Their  forms  are  similar  because 
of  the  habit  of  their  slender  branches,  which  arise  from  near  the  base 
and  reach  obliquely  upward,  producing  the  effect  of  an  inverted  cone. 
They  are  both  taller  than  guayule,  but  the  shade  cast  by  them  is  small 
in  amount,  and  less  is  cast  by  the  ocotillo  than  by  the  gobernadora.  The 
only  places  where  the  ocotillo  grows  thickly  are  in  certain  situations  on 
south  slopes,  and  here  it  often  forms  a  dense  thicket.  When  thickly  grow- 
ing it  would  interfere  with  the  rapid  harvesting  of  guayule  because  of  the 
thorny  branches,  but,  excepting  for  the  draft  it  makes  on  the  soil  for 
water,  the  effect  upon  guayule  is  negligible.  This  applies  about  equally 
to  gobernadora,  which  in  North  Zacatecas, however,  reproduces  itself  quite 
rapidly  by  seed,  and  so  may  readily  come  to  occupy  too  much  ground. 

PALMA  SAMANDOCA  (SAMUELLA  CARNEROSA)  AND  SOTOL  (DASYLIRION  CEDROSANUM). 

These  are  similar  in  form.  Each  plant  has  a  single  stem  supporting  a 
large  rosette  of  leaves.  The  sotol,  however,  rarely  rises  sufficiently  above 
the  surface  of  the  soil  to  free  the  surface  from  the  lower  dead  leaves, 
which  cover  about  10  square  feet  of  area.  Both  plants  are  valuable  eco- 
nomically, the  palma  samandoca  affording  a  fiber  of  less  value  than  the 
lechuguilla,  but  of  which  a  good  deal  is  prepared,  while  the  other  is  the 
basis  for  the  manufacture  of  the  whisky -like  liquor,  mescal  sotol,  or  simply 
sotol.  Neither  of  these  occurs  in  sufficient  numbers  to  figure  in  compe- 
tition with  the  guayule  within  its  proper  habitat.  Indeed,  for  reasons  not 
yet  understood,  when  sotol  grows  densely,  forming  a  chaparral,  guayule  is 
entirely  absent.  One  reason,  if  not  the  only  important  one,  is  that  the 
sotol  appears  not  to  be  confined  to  limestone  areas,  but  is  not  excluded 
from  them. 


The  Environment.  39 

SANGRE  DE  DRAGO  QATROPHA  SPATULATA). 

This  plant  is  a  very  characteristic  xerophyte,  and  is  found  beyond 
the  limits  of  the  Chihuahuan  desert,  westward  into  Sonora  (MacDougal, 
1908).  The  upper  part  of  the  plant  consists  of  a  simple,  dark-brown 
and  somewhat  fleshy  stem,  scarcely  branched  at  all  and  slightly  curved. 
The  leaf- producing  lateral  shoots  are  very  short,  and  are  roughened  with 
small  scales;  from  them  arise  the  bright  green  narrow  leaves  in  clusters. 
Reproduction  takes  place  readily  by  means  of  seed,  and  the  plant  spreads 
by  underground  stems  which  are  thick  and  fleshy,  and  are,  in  fact,  water- 
storage  organs.  Like  the  lechuguilla  it  is  a  colonial  form,  growing  in 
dense  patches,  but  is  less  able  to  occupy  the  ground  to  the  exclusion  of 
other  plants  because  of  the  slender  aerial  parts.  Its  ability  to  take  up 
large  amounts  of  water  from  the  superficial  soil  must,  however,  be  reck- 
oned with.  There  is  little  doubt  that  this  is  a  dominating  type. 

RASTRERO  (OPUNTIA  MEGALARTHRA). 

This  is  a  spreading,  low  form  of  prickly  pear.  Though  sometimes 
very  densely  packed,  making  progress  difficult,  mechanically  it  interferes 
comparatively  little  with  guayule.  This  is  to  be  explained  by  the  fact 
that, on  account  of  the  edgewise  position  of  the  flat,  procumbent  branches, 
very  little  soil  surface  is  actually  occupied.  One  finds,  indeed,  that  young 
plants  of  guayule  are  frequently  abundant  in  irregular  rows  beneath,  or 
nearly  so,  the  branches  of  the  opuntia.  It  is  not  unlikely  that  the  spines 
of  the  former  aid  somewhat  in  protecting  the  guayule  from  jack-rabbits 
and  other  predatory  animals,  and  so,  in  this  particular  respect,  help  it 
along  rather  than  hinder  it.  While  this  opuntia  is  a  persistent  type,  its 
occupancy  of  the  ground  is  apparent  rather  than  real. 

A  composite  shrub  (Zexmenia  brevifolid) ,  huisache  ( Acacia  farnesi- 
and) ,  gatufio  (Acacia  greggii) ,  and  asafran  (Buddleia  marrubiifolia)  are  all 
shrubby,  freely  branching  kinds.  The  last  resembles  guayule  in  color, 
and  the  novice  may  easily  mistake  the  one  for  the  other.  The  gatufio 
and  huisache  are  small  trees  with  slender  branches,  and  make  but  little 
shade.  The  nature  of  the  competition  between  these  forms  and  lechuguifla 
is  more  evident  than  in  the  case  of  these  and  guayule.  They  are  slow- 
growing  and  do  not  reproduce  themselves  except  by  seed,  and  this  not 
rapidly.  Nevertheless,  excepting  the  gatuno,  they  may  be  found  growing 
very  plentifully  in  some  situations  and  often  outnumber  the  guayule. 
Thus  on  north  slopes  the  composite  shrub  is  frequently  more  numerous 
than  the  guayule. 

PEYOTE  (PEYOTL)   (LOPHOPHORA  WILLIAMSII  AND  L.  LEWINII). 

These  cacti  are  the  mescal-buttons  or  dry  whisky  of  the  Texas  In- 
dians and  cow-men,  and  have  been  sought  after  as  the  source  of  a  little 
understood  alkaloid  of  marked  effects  upon  the  nervous  system.  The 
exposed  part  of  the  plant  is  little  more  than  a  convex  disk  a  few  centi- 
meters in  diameter,  of  fleshy  texture.  The  stem  and  root  together  form 
a  conical,  fleshy  mass.  They  are  a  very  modest  element  in  the  vegetation, 
occupying  little  surface,  and  may  be  disregarded  from  a  practical  point 
of  view. 


Guayule. 


There  can  be  little  doubt  that  the  component  elements  in  such  a  veg- 
etation are  in  a  state  of  ebb  and  flow,  and,  in  view  of  the  density  of  the 
vegetation,  in  contrast  with  the  condition  usually  met  with  in  deserts,  con- 
stitute an  important  question  economically.  Here  the  individuals  come 
into  actual  contact  above  ground,  where  the  competition  is  often  severe, 
as  well,  presumably,  as  below  ground.  Referring  especially  to  guayule, 
it  may  be  accepted  that,  when  a  plant  is  once  well  started,  it  is  seldom 
killed  outright  by  contact  with  its  neighbors,  but  the  occupancy  of  the 
ground  by  other  species  which  have  superior  methods  for  spreading  grad- 
ually reduces  the  available  surface  and  water-supply  for  the  guayule.  This 
plant  takes  advantage  of  surface-water  by  means  of  its  superficial  roots 
and  plants  with  which  it  is  associated  and  which  behave  similarly  (e.g., 
Jatropha  spatulata)  must  come  into  severe  competition  with  it  in  this 
regard.  But,  assuming  that,  for  purposes  of  forestry,  it  is  desirable  to 
thin  out  other  vegetation  in  order  to  favor  the  guayule,  the  question 
arises  as  to  the  effect  upon  the  germination  of  seed  of  this  plant,  which  is 
undoubtedly  favored  by  partial  shade.  It  may  be  argued  that  the  superior 
numbers  of  seed  available  and  the  shade  of  the  guayule  plants  themselves 
will  suffice,  and  this  seems  probable.  On  the  germination  of  seed  in  the 
open  more  will  be  said,  based  upon  experimental  evidence  (Chapter  IV). 
Denuded  areas  are  under  observation,  and  the  future  may  be  expected  to 
bring  exact  observation  to  bear  upon  the  practical  question  of  the  value 
of  clearing  land,  as  well  as  upon  the  theoretical  aspect  of  the  questions 
above  stated.  (See  also  Chapter  IX.) 

PARASITISM. 

Of  vegetable  parasites  affecting  the  guayule  only  two  are  at  present 
known.  Of  lesser  importance,  so  far  as  we  may  judge,  is  a  rust  hitherto 
known  as  Uredo  parthenn  Speg.  (fig.  5).  Prof.  J.  C.  Arthur,  to  whom 
material  was  sent  for  identification  in  April,  1908,  reports  that  the  fungus 
properly  belongs  in  the  genus  Puccinia,  and  may  be 
called  Puccinia  parthenn  (Speg.)  Arthur,  ined.,  for  the 

O\  ((       ^        purpose  of  record. 
J  \\       //  It  has  been  noticed  that  the  fungus  appears 

chiefly  on  plants  which  are  on  the  north  slopes  of  ar- 
royos,  especially  near  the  bottom,  where  the  relative 
humidity  is  most  favorable,  since  it  is  here  that  the 
highest  vapor-tension  exists.  It  has  been  found  also 
on  plants  growing  on  ridges,  and  especially  on  those 
which  are  subject  to  a  condition  which  we  have  called 
"witches'  broom,"  in  which  the  leaves  are  small  and 
very  much  crowded.  It  appeared  in  the  spring  of 
1908  also  on  plants  which  had  been  grown  under 
irrigation  at  Cedros,  apparently  on  the  older  leaves, 
which  still  remained  attached  from  the  previous 
year.  The  parasite  is  not  at  all  plentiful,  and  appears 
to  be  absent  almost  entirely  from  guayule  growing  in  open  situations.1 

1  A  small  seedling  which  germinated  in  the  early  summer  of  1908  was  found 
in  April  1909  with  a  single  infection  spot,  quite  in  the  open  foot-slope  (Station  3), 
in  which  situation  the  fungus  is  seldom  seen. 


FIG.  5. — Teleuto  and  ure- 
dospores  of  Puccinia 
parthenii  (Speg.)  Ar- 
thur. 


The  Environment.  41 

Of  more  importance,  economically,  is  the  "seda"  (silk)  or  dodder 
(Cuscuta  sp.) ,  which  often  grows  very  plentifully.  The  habit  of  this  par- 
asite is  well  known,  so  that  no  account  of  the  plant  is  here  necessary.  It 
is  very  readily  recognized  as  a  yellow  or  orange  vine-like  leafless  organ- 
ism which  winds  about  the  upper  twigs  and  leaves  of  the  host.  It  is  not 
confined  to  the  guayule,  being  found  also  on  hojasen  (Flourensia  cernua), 
on  mariola  (Parthenium  incanum) ,  on  tatalencho  (Gymnosperma  corym- 
bosum),  and  other  perennial  plants,  and  probably  on  some  summer  an- 
nuals. It  reproduces  itself  by  means  of  seeds  which  germinate  after  the 
advent  of  the  summer  rains,  but  is  to  be  found  vegetating  vigorously  long 
before  this  time.  This  is  explained  by  the  fact  that  it  passes  the  win- 
ter in  rather  tight,  compact  clusters  of  thread-like  stems,  tightly  wound 
about  the  uppermost  twigs  and  leaves  of  the  host.  (Lloyd,  19080!.)  Thus 
it  is  independent  of  seed  and  is  a  true  perennial.1 

The  effect  of  the  dodder  upon  the  guayule  is  due  to  two  causes. 
It  diverts  water  and  foods  [from  the  host  into  its  own  tissues  and  thus 
reduces  the  rate  of  growth,  and  it  strangulates  the  twig  and  leaves  upon 
which  it  fastens  itself.  There  is  thus  produced  a  dwarfing  and  distortion 
which  is  reflected  in  the  whole  habit  of  the  plant. 

As  soon  as  growth  commences  in  the  host,  the  dodder,  which  is 
ready  at  the  top  of  the  previous  year's  growth  to  take  hold  of  the  new 
tender  tissues,  begins  to  twine  about  the  newly  forming  stem  and  leaves 
and  soon  overtakes  and  strangulates  them.  The  effect  is  to  produce  very 
slowly  growing  plants,  and  it  is  seen  that  the  presence  of  much  dodder 
would  materially  reduce  the  annual  accretion  of  growth  and  therefore  of 
rubber.  In  periods  of  severe  drought  the  effect  of  the  dodder  is  even  more 
marked,  since  it  diverts  the  already  meager  water-supply  and  thus  causes 
the  death  of  the  portion  of  the  twig  at  and  above  the  zone  at  which  the 
dodder  is  found.  Plants  with  twigs  killed  in  this  way,  and  in  which 
the  dodder  itself  had  succumbed,  were  found  at  the  close  of  a  sustained 
drought,  in  April  1909.  The  dodder  should  therefore  be  stamped  out 
wherever  it  may  be  found.  The  best  and  only  practical  means  is  to  har- 
vest with  the  initial  crop  all  the  guayule  affected  with  the  parasite.  In 
this  way  the  parasite  will  be  checked,  and  additional  checks  will  be  re- 
ceived at  each  harvesting  by  following  the  same  rule. 

Indications  of  another  vegetable  parasite  were  thought  to  be  seen  in 
the  "witches'  broom"  above  mentioned,  but  material  examined  by  Prof. 
W.  G.  Farlow  gave  no  clue  to  the  cause.  The  densely  packed  leaves  in- 
deed favor  the  growth  of  the  rust  already  described,  but  this  is  quite  a 
secondary  condition.  It  is  possible  that  the  distortion  is  due  to  the  crop- 
ping of  the  guayule  by  animals,  but  not  all  plants  so  treated  show  it,  else 
nearly  all  would  be  affected.  Plants  closely  in  the  field  trimmed  back 
(Station  2,  quadrats  i,  2)  show  a  tendency  to  produce  "witches'  broom," 
indicating  that  constant  or  close  browsing  by  animals  may  after  all  be 
the  cause  of  this  condition. 


1  Cuscuta  is  sometimes  a  perennial  as  far  north  as  the  State  of  New  York. 
Stewart  et  al.,  Bull.  305,  Agri.  Exp.  Sta.  N.  Y.,  Nov.,  1908. 


42  Guayule. 

ANIMAL  PARASITES. 

The  root-system,  particularly  the  tap-root  and  its  larger  branches, 
are  frequently  found  to  be  infested  with  two  species  of  the  Coccidae,1  Cero- 
puto  yucca  (Coq.),  and  a  species  of  Orthezia,  distinguishable  from  the  for- 
mer by  the  fluted,  waxy  egg-case  attached  to  the  abdomen.  The  number 
of  these  insects  found  on  plants  in  the  field  is  not  inconsiderable,  and  may 
be  responsible  for  lesions  in  the  root-tissues  which  affect  the  growth  of 
the  plant.  But  of  more  importance  is  the  circumstance  that  they  occur 
in  greater  numbers  upon  seedlings  raised  under  cultural  conditions  in 
wooden  trays.  Plantlets  a  few  centimeters  in  height  have  been  found 
with  a  dozen  or  more  large  individuals  on  the  tap-root,  the  diameter  of 
which  was  not  as  great  as  the  breadth  of  the  mature  insects.  They  may 
therefore  easily  be  responsible  for  retardation  of  growth,  though  external 
evidence  of  lesions  has  not  been  noted. 

Field  plants  especially  are  often  infested  below  the  surface  of  the 
soil  by  a  scale,  identified  by  Dr.  C.  L.  Marlatt  as  Targionia  dearnessi  Ckll. 
This  is  a  widely  distributed  species  in  this  country.  Large  tap-roots  are 
frequently  half  covered  by  this  parasite. 

A  gall  insect  attacks  the  leaves  and  inflorescence.  The  female  punc- 
tures the  young  leaves  and  stems,  the  peduncles,  and  even  the  bracts  of 
the  capitula,  and  the  resulting  galls  produce  marked  distortion.  Many  of 
the  affected  leaves  fail  of  anything  approaching  normal  development ;  the 
peduncles  are  hypertrophied  unevenly  and  become  very  much  contorted, 
and  the  inflorescence  fails  to  develop.  The  net  result  of  the  work  of  this 
insect  is  to  reduce  the  rate  of  growth  very  materially  and  to  cause  a  prac- 
tically complete  abortion  of  the  flowers  and,  therefore,  of  the  seed.  The 
plants  affected  are  readily  recognized  on  account  of  the  irregularity  and 
lumpiness  of  the  terminal  growths.  The  stems  proper  do  not  seem  to  be 
affected,  as  the  insect  appears  to  commence  its  work  toward  the  close 
of  the  season  of  growth  and  to  confine  itself  to  the  last-formed  leaves, 
which  remain  attached  throughout  the  winter,  and  to  the  inclosed  young 
inflorescences.  The  increase  of  growth  in  the  stem  is,  however,  affected 
indirectly,  and  the  annual  accretions  frequently  amount  to  less  than  i 
cm.,  and  scarcely  ever  to  more  than  2  cm.,  during  the  period  of  attack. 
Many  plants  in  circumscribed  areas  are  subject  to  the  attacks  of  these 
insects,  and  it  may  readily  become  a  serious  menace  to  both  the  growth 
of  the  plants  and  to  their  seeding  power.  The  following  notes  have  been 
kindly  furnished  me  by  Dr.  Mel.  T.  Cook: 

The  study  of  this  material  presented  many  difficulties,  as  must  necessarily  be 
the  case  when  it  is  not  possible  to  make  a  field  study. 

A  gall  produced  by  Cecidomyia  parthenicola  on  Parthenium J  in  New  Mexico  has 
been  described  by  T.  D.  A.  Cockerell  in  Entomologist,  July,  1900,  p.  201.  The  gall 
before  me  does  not  fully  correspond  with  Cockerell's  species,  and  yet  I  should  hesi- 
tate to  say  that  it  is  an  entirely  different  species  without  further  study,  which  is  im- 
possible with  the  material  in  hand.  Dissection  of  the  material  showed  two  entirely 
different  species  of  larva  and  immature  insects,  cecidomyid  and  cynipidous,  while  a 
study  of  the  histology  presented  certain  confusing  and  anomalous  characters. 

1  Kindly  determined  for  me  by  Mr.  T.  G.  Sanders,  through  the  courtesy  of 
Dr.  L.  O.  Howard. 

2  Parthenium  incanum,  presumably. 


The  Environment.  43 

The  isolated  galls  were  small,  monothalamous,  and  in  the  shape  of  a  truncated 
cone,  usually  on  the  upper  surface  of  the  leaves  and  standing  in  an  oblique  position 
The  opening  of  the  larval  chamber  was  through  the  top  and  was  guarded  by  hair- 
like  growths  or  trichomes,  which  pointed  inward.  This  would  indicate  a  cecidomyid 
gall,  but  certain  preparations  showed  the  opening  closed  by  a  thin  membrane. 
Whether  this  latter  condition  was  real,  therefore  proving  the  presence  of  two  species 
of  galls,  or  only  apparent,  was  difficult  to  determine,  owing  to  a  tendency  of  the  galls 
to  coalesce,  forming  irregular  masses. 

HISTOLOGY. 

The  gall  in  its  earliest  state  shows  the  reduction  of  the  palisade  into  cells  of 
the  mesophyll  type.  This  condition  is  characteristic  of  the  origin  of  all  leaf  galls. 
As  the  gall  develops,  the  cells,  which  constitute  the  lining  of  the  larval  chamber,  are 
rich  in  protoplasmic  content,  which  decreases  from  inner  to  outer  surface.  This  is 
indicated  very  readily  by  the  stains  and  is  characteristic  of  the  more  highly  devel- 
oped galls  and  usually  designated  as  the  nutritive  zone.  A  little  later  certain  galls 
showed  a  reduction  of  the  nutritive  zone  and  the  formation  of  a  protective  zone  of 
sclerenchyma  cells  just  outside  the  nutritive  zone.  The  presence  of  this  protective 
zone  is  characteristic  of  the  galls  produced  by  cynipidous  insects,  and  the  writer  has 
never  found  them  in  galls  caused  by  cecidomyid  insects. 

From  the  above  facts,  it  appears  that  we  may  have  two  species  of  galls,  one  pro- 
duced by  a  cynipidous  insect  and  the  other  by  a  cecidomyid,  or  a  single  gall  which 
has  been  parasitized. 


FIG.  6.— The  guayule  barkbeetle  (Pityophthorus  mgricans  Bland),    (a)  Work  of  beetles  and  larva  in 
bark  and  wood.   (6)  Adult  beetle,  greatly  enlarged.  Small  figure  at  right  shows  natural  size    (c)  Egg- 
galleries  of  parent  beetles,  with  intervening  larval  mines,  all  grooved  on  surface  of  wood.     (. 
illustrations  loaned  by  the  Bureau  of  Entomology,  U.  S.  Dep.  Agric.) 


Guayule. 


THE  GUAYULE  BORER. 

In  the  fall  of  1907  it  was  noticed  that  guayule  in  the  stack  (plate 
4,  fig.  A) ,  awaiting  treatment  for  the  extraction  of  the  rubber,  was  being 
attacked  by  an  insect,  the  only  signs  of  which  were  the  finely -powdered 
debris  escaping  from  minute,  circular  openings  in  the  bark.  It  was  at 
once  evident  that  a  borer  of  some  kind  was  at  work.  Material  was  sent 
to  Dr.  L.  O.  Howard,  who  kindly  referred  the  matter  to  Dr.  A.  D.  Hopkins, 
in  charge  of  forest  insect  investigations,  Bureau  of  Entomology,  U.  S. 
Department  of  Agriculture,  to  whom  I  am  indebted  for  the  accompanying 
notes  and  drawings  (fig.  6,  p.  43).  Dr.  Hopkins  writes  as  follows: 

The  beetle  is  Pityophthorus  nigricans  Bland.  It  has  also  been  reported  to  the 
Bureau  of  Entomology  by  H.  Pittier,  who  found  it  injuring  the  same  plant  at  Tor- 
re6n,  Coahuila,  Mexico.  The  insect  is  of  special  interest  because  of  its  habit  of  attack- 
ing a  plant  of  such  commercial  value,  and  on  account  of  its  being  the  largest  repre- 
sentative of  the  division  of  the  genus  to  which  it  belongs.  Those  of  one  division 
infest  coniferous  trees  only,  while  those  of  the  other,  to  which  this  species  belongs, 
infest  only  the  broad-leaved  plants  and  trees.  The  guayule  barkbeetle  evidently 
attacks  the  plant  after  it  is  dead,  or  soon  after  it  has  been  cut,  and,  as  has  been 
shown  by  the  specimens  in  the  forest-insect  collection  of  the  Bureau  of  Entomology, 
may  continue  to  breed  in  the  same  bark  and  wood  for  several  years.  It  is  evident 
that  the  prompt  utilization  of  the  plant  for  the  manufacture  of  rubber  within  a  few 
days  after  it  is  cut  would  prevent  all  losses  from  this  source. 

Inasmuch  as  the  buyers  of  shrub  sometimes  accumulate  large  quan- 
tities and  place  it  in  stacks  until  needed,  and  as  this  may  represent  large 
investments,  the  amount  of  damage  may  represent  no  inconsiderable 
loss.  In  order  to  determine  what  this  loss  might  amount  to,  a  piece  of 
stem  of  average  thickness  which  had  been  attacked  by  the  borer  was 
weighed  as  a  whole.  It  was  then  decorticated  and  the  insect  debris  was 
carefully  removed.  Some  of  the  debris  had  of  course  been  lost,  and  thus 
an  error  is  introduced  into  the  calculation  of  fully  5  per  cent  of  the  total 
weight  of  the  bark.  The  tunneling  done  by  the  insect  was  not  complete, 
however,  and  for  this  reason  the  figures  may  be  regarded  as  the  average 
result  of  the  damage  which  may  occur  in  the  space  of  a  month  or  two. 


TABLE  19. 


Length  of  sample  piece  of  stem  ....25. 

Diameter  of  the  wood 9.8 

Thickness  of  the  bark 2.3 

Total  diameter  of  the  stem 14.4 


Weight  of  the  whole 

Weight  of  wood  cleaned  of  debris . 
Weight  of  bark  cleaned  of  debris . 
Weight  of  material  lost  (with 

probable  correction) 

Of  which  half  is  bark,  viz o  .  i 


grams 
3.801 

i-7°3 
1.903 

O.  2 


It  can  be  mathematically  shown  that  the  amount  of  destruction  in 
the  smaller  twigs  in  which  the  insects  work  may  amount  to  very  con- 
siderably more,  indeed  to  the  extent  of  40  per  cent  of  the  volume  of  the 
bark  (cortex).  Inasmuch  as  the  bark  contains  practically  all  the  rubber, 
it  is  seen  that  the  loss  may  be  great  enough  to  warrant  serious  considera- 
tion. It  must  be  observed,  however,  that  the  comminution  of  the  cor- 
tical tissues  by  the  beetle  does  not  diminish  the  amount  of  rubber  in 
the  stem  except  by  the  amount  that  happens  to  escape  through  the  en- 
trances, so  that  the  real  question  is,  whether  the  comminution  of  the  cor- 


The  Environment.  45 

tex  and  of  the  rubber  contained  in  it  renders  the  rubber  unavailable  in 
the  manufacture  of  the  crude  product  or  not.  In  order  to  answer  this 
question,  a  sufficient  quantity  of  the  debris  was  collected  and  subjected 
to  mastication.  By  this  means  it  was  possible  to  cause  the  partial  agglom- 
eration of  the  rubber,  but  it  was  quite  impossible  to  separate  out  the 
"bagasse"  on  account  of  the  fineness  of  the  particles.  These  have  the 
effect  of  separating  the  rubber  so  that  it  is  in  the  form  of  a  fine  mesh- 
work,  the  connecting  isthmuses  not  appearing  to  be  great  enough  to 
overcome  the  surface-tension  of  the  smaller  masses.  Microscopical  ex- 
amination shows  the  mass  to  be  composed  of  minute  fragments  of  tissue 
derived  from  the  wood  and  cortex  embedded  in  the  rubber.  Measure- 
ments of  these  particles  showed  them  to  be  0.02  to  o.i  mm.  in  size,  occa- 
sional pieces  being  as  large  as  0.5  mm.  If  during  mastication  one  is 
careful  to  allow  only  a  small  amount  of  saliva  to  bathe  the  mass,  it  may 
be  held  together  for  some  time,  but  if  it  be  flooded  for  a  moment  and 
worked  meanwhile,  it  will  quickly  disintegrate  and  can  not  be  reagglom- 
erated.  It  therefore  appears  that  the  work  of  the  beetle,  while  not  destroy- 
ing the  rubber,  puts  it  into  such  condition  that  it  is  lost  to  the  manu- 
facturer who  uses  a  mechanical  method  of  extraction,  since  the  minute 
particles  can  not  be  made  to  agglomerate.  When  the  insects  have  once 
got  a  fair  start  in  a  stack-yard  the  amount  of  damage  which  may  be 
caused  in  a  short  time  by  their  very  large  numbers  may  be  great  enough 
to  warrant  the  adoption  of  means  to  avoid  the  loss,  if  it  is  found  that 
stacking  the  guayule  is  necessary. 

CROPPING  BY  GRAZING  ANIMALS. 

It  has  been  pointed  out  that  the  growing  guayule  is  browsed  by  an- 
imals. Burros,  jack-rabbits,  cotton-tails,  and  goats  are  all  given  to  this, 
and  as  these  animals  are  numerous  a  great  loss  is  entailed.  Goats  are 
herded  habitually  in  the  guayule  fields,  and  these  animals,  with  their 
all-devouring  appetites,  eat  almost  everything  that  grows.  Not  the  least 
damage  done  by  them  is  the  wholesale  destruction  of  the  developing 
shoots  and  flower-buds,  reducing  the  crop  of  seeds  very  greatly.  Goats 
and  burros  may,  however,  be  pastured  away  from  the  guayule  fields,  and 
thus  loss  may  be  avoided. 

The  work  of  rabbits,  where  other  food  is  available,  is  not  serious, 
though  in  the  event  of  adopting  forestry  methods  they  may  become  a 
menace  to  the  plant.  These  marauders  do  not  merely  crop  off  the  foliage 
and  new  shoots;  they  lop  off  whole  branches,  which  are  left  on  the  ground 
to  die.  One  jack-rabbit  may  therefore  do  a  great  deal  more  damage  than 
a  goat  in  the  same  time.  It  has  been  noticed  that  they  treat  the  gober 
nadora  in  the  same  way.  One  frequently  sees  a  complete  circle  of  dead 
branches  about  the  base  of  a  bush,  all  having  been  lopped  off  at  one  time. 


. 


CHAPTER  III. 
DESCRIPTION  OF  THE  GUAYULE, 

Parthenium  argentatum  Gray. 


SEED. 

The  word  seed  is  here  applied  in  a  loose  sense,  inasmuch  as  the  body 
to  which  the  term  is  applied  is,  correctly  speaking,  an  achene,  a  one- 
seeded  fruit  in  which  the  pericarp  remains  indehiscent  and  dry.  What 
passes  as  "seed"  in  guayule  is  a  mixture  of  achenes,  sterile  flowers, 
involucral  scales,  and  pedicels,  and,  inasmuch  as  the  opportunities  for 
sophistication  are  nearly  always  at  hand,  and  for  the  reason  that  the 
peon  employed  for  the  gathering  of  seed  will  not  always  be  diligent  in 
distinguishing  between  guayule  and  mariola  "seed,"  the  present  chapter 
may  appropriately  begin  with  a  description  of  the  flower. 

In  the  genus  Parthenium,  as  in  all  the  Compositae,  the  order  to  which 
it  belongs,  the  flowers  are  arranged  in  heads  or  capitula  (fig.  10).  In  the 

guayule  these  are  about  5  mm.  in 
diameter,  and  contain  two  kinds 
of  flowers,  commonly  known  as 
ray  and  disk  flowers.  The  rays 
are  normally  five  in  number,  and 
are  readily  recognized  during 
flowering  by  the  open  corollas, 
which  project  radially  beyond 
the  margin  of  the  capitulum. 
These  only  produce  seed,  each  a 
single  one,  if  fertile.1  The  disk 
flowers  produce  pollen  but  are 
incapable  of  setting  seed,  al- 
though the  pistil  is  present  and 
serves  after  the  fashion  of  a  pis- 
ton to  eject  the  pollen,  as  commonly  occurs  in  the  Compositae.  When  the 
fruit  is  ripe  and  the  period  of  flowering  is  quite  past,  the  capitulum 
becomes  dismembered  in  a  somewhat  unusual  fashion.  Each  ray-flower, 
the  two  adjacent  disk  flowers  and  their  subtending  involucral  bracts, 
become  attached  to  each  other  by  concrescence,  and  fall  away  as  a  whole 
(fig.  7).  The  remainder,  i.e.,  all  but  ten  of  the  disk  flowers,  also  remain 
attached  to  each  other  and  fall  away  as  a  shriveled,  conical  mass.  There 
remain  behind  five  involucral  bracts  persistently  attached  to  the  recep- 
tacle which  supported  the  whole.  In  collecting  "seed"  all  of  these  are 
taken,  so  that  it  will  be  seen  that  the  bulk  of  the  material  is  chaff. 

Considering  the  fertile  flower  and  its  accompaniments,  we  observe 
that  the  achene  is  hidden  between  the  adjoined  pair  of  disk  flowers  and 
its  own  bract.  This  bract,  which  is  quite  broad  and  concavo-convex,  is 


FIG.  7. — Ray-flower  with  attached  disk  flowers  and 
the  subtending  bracts.     Parthenium  incanum. 


46 


Polyembryony  occasionally  occurs. 


Description  of  the  Guayule. 


47 


composed  of  three  morphological  elements,  fused  above,  but  more  or  less 
loosely  connected  below;  a  rare  occurrence,  analogous  to  the  condition 
of  some  stamens.     The  middle  element 
is  the  narrowest,  and  is  the  bract  proper 
of  the  pistillate  flower.    This,  to  be  seen, 
must  be  dissected  out. 

Another  peculiar  feature  then  be- 
comes apparent,  namely,  that  the  two 
disk  flowers  can  not  be  separated  from 
the  achene  without  pulling  away  two 
narrow  strips  of  tissue  from  its  margins. 
(Fron  et  Francois,  1901.)  The  whole 
arrangement  would  appeal  to  the  tele- 
ologist  as  an  excellent  adaptation  for 
dissemination  by  the  wind  or  by  water, 
since  the  thin,  light,  and  air-imprison- 
ing tissue  may  serve  as  wings  or  floats 
according  to  circumstances.  The  achene 
itself  is  crowned  by  the  persistent  but 
shriveled  corolla,  and  at  either  side  of 
this  and  against  its  ventral  (upper  or 
inner)  aspect  are  three  short  awns,1  one 
in  each  position,  The  achene  proper 
is  ovate,  with  an  acute  base.  It  is  par- 
tially clothed  with  short  appressed  hairs, 
but  for  which  the  pericarp  would  be 
black  or  dark  gray.  The  achene  meas- 
ures 2.5  mm.  in  length  by  1.8  in  breadth 
when  of  normal  size,  exclusive  of  the 
awns. 

The  "seed"  of  the  two  other  spe- 
cies, mariola  (P.  incanum)  and  P.  hys- 
terophorus2  (an  annual),  which  grows 
with  or  near  the  guayule,  may  be 
distinguished  by  attention  to  the  char- 
acter of  the  lateral  awns,  which  may 
readily  be  seen  with  a  lens  by  viewing 
them  as  they  project  beyond  the  bract. 
In  the  guayule  the  awns  are  brown, 
with  papery,  denticulate  margins.  In 
the  mariola  these  are  slender,  appearing 
denticulate  or  quite  without  membra- 
nous margins,  tapering  and  distinctly 
reflexly  curved,  and  are  usually  darker  in  color,  being  black  toward  the 

1  Taxonomic  works  usually  indicate  that  there  are  only  2  awns,  but  this  is 
an  error.  There  are  3  awns  in  Partlienium  argentatum  and  P.  incanum;  2  in  the 
herbaceous  P.  lyratum  and  P.  hysterophorus.  Engler  and  Prantl  describe  the  genus 
as  having  2  to  3  awns,  but  do  not  indicate  further  details. 

1  This  plant  grows  in  great  profusion  in  the  summer  months  in  the  alluvial 
plains  upon  which  the  guayule  lands  border. 


FIG.  8.  —  A,  a  fully  germinated  seedling  of 
guayule,  before  induration,  X  8  ;  B,  cotyle- 
dons and  first  two  foliage  leaves  ;  C,  trans- 
verse section  through  achene  of  a  ray  flower 
and  its  two  attached  disk  flowers. 


48 


Guayule. 


base.  In  P.  hysterophorus  and  P.  lyratum  they  are  very  broad,  and  are 
membranous  in  the  former.  Figs.  7  and  9  will  make  these  and  other 
characters  evident. 


FIG.  9. — Achenes  of  (i)  Parthenium  argentatum,  (2)  P.  hysterophorus,  (3)  P.  incanum,  (4)  P.  lyratum. 

SEEDLING. 

When  germination  is  complete  the  seedling  of  the  guayule  consists 
of  a  short  primary  stem  (hypocotyl),  5  to  10  mm.  in  length,  terminating 
in  a  long,  slender  tap-root.  Attempts  to  find  the  end  of  this  in  the  field 
have  been  fruitless,  on  account  of  the  nature  of  the  ground  and  because 
of  its  very  tender  character  and  great  length  and  thinness.  Experiments 
show  that  it  reaches  a  depth  of  at  least  several  inches.  This  slender  root- 
let, with  very  few  branches,  is  the  means  of  keeping  the  plantlet  supplied 
with  water  from  the  soil  for  some  months,  as  frequently  during  the  first 
year  in  the  field  no  adequate  development  of  lateral  roots  occurs.  The 
seed  leaves  (cotyledons)  are  nearly  or  entirely  circular  in  form,  and  range 
in  size  from  2.5  mm.  in  width  by  3  mm.  in  length  to  4.5  mm.  in  width 
and  4.7  mm.  in  length,  according  to  various  conditions.  At  the  apex  of 
each  cotyledon  is  a  hydathode,  composed  of  a  group  of  water  stomata. 
Other  conditions  being  the  same,  seedlings  grown  in  the  shade  and  high 
relative  humidity  have  the  largest  cotyledons  (plate  34,  figs.  6,  9),  and 
the  largest  were  seen  on  seedlings  grown  experimentally  under  such  con- 
ditions. The  primary  stem  is  about  i  mm.  in  diameter,  and  in  seedlings 
grown  under  natural  conditions,  i.e.,  with  direct  sunlight,  is  dark  red;  in 


A.  The  root-system  of  guayule. 

B.  Group  of  plants  which  started  as  retonos. 

C.  A  strongly  monopodial  retofio. 


Description  of  the  Guayule.  49 

shade  plants  it  is  green.  The  dark-red  color  extends  also  over  the  under 
surface  of  the  cotyledons,  which  are  rather  thick  in  sun  forms,  and  thin- 
ner in  shade-grown  plantlets  (plate  34,  figs.  4,  6). 

The  early  foliage  leaves,  soon  after  germination  and  because  of  the 
very  short  internodes,  are  closely  crowded.  By  partial  etiolation  these 
internodes  may  be  caused  to  lengthen,  and  thus  the  structure  of  the  pri- 
mary epicotyledonary  stem  may  be  better  studied.  In  this  way  points 
may  be  made  clear  which  otherwise  would  with  difficulty  be  explained. 
The  first  8  leaves  are  usually  ovate,  entire,  slightly  acute,  and  taper 
suddenly  into  the  petiole  (fig.  8).  They  are  clothed,  as  are  all  the  foliage 
leaves,  with  closely  set  T-shaped  hairs  (plate  30,  figs.  9-11)  laid  parallel 
to  the  axis  of  the  leaf,  and  thus  is  produced  that  light  green-gray,  satiny 
sheen  which  characterizes  the  plant.  The  first  leaf  is  usually  about  i  cm. 
long  by  3  mm.  wide,  though  measurements  vary  a  good  deal.  In  the  mar- 
iola  seedling  the  earliest  leaves  resemble  those  of  the  guayule,  but  differ  in 
being  broader  and  lanose,  a  difference  due  to  the  form  of  the  trichomes, 
those  of  mariola  being  of  the  "whip  "  form  found  frequently  in  the  Com- 
positae.  As  the  hairs  are  much  thicker  on  the  under  side  of  the  leaf,  the 
species  may  be  very  readily  recognized  even  when  only  one  foliage  leaf 
has  developed,  though  identification  is  difficult  before  this  leaf  appears. 

The  last  formed  of  the  entire-margined  seedling  leaves  may  reach, 
in  field  plants,  a  length  of  7  to  8  cm.  and  a  width  of  1.5  cm.  The  first 
approach  to  the  mature  leaf  form  is  seen  in  a  single  tooth,  usually  on 
one  margin  only,  at  about  the  middle  of  the  blade.  In  the  next  stage 
the  tooth  may  be  found  on  both  sides,  and  larger,  while  half-way  between 
their  position  and  the  apex  a  second  pair  of  teeth  appears.  By  basal 
contraction  of  the  blade  and  extension  of  the  upper  portion,  the  first 
teeth  appear  to  move  downwards,  and  by  enlarging  attain  lobate  pro- 
portions. The  leaf  is  now  relatively  shorter  and  broader.  An  additional 
pair  of  basal  teeth  may  also  add  to  the  complexity.  While  this  descrip- 
tion, illustrated  well  in  plate  18,  is  generally  true,  few  plants  are  more 
variable  as  regards  the  form  of  the  leaf  than  the  guayule,  and  this  varia- 
bility is,  with  the  exception  of  the  earliest  foilage  leaves  to  be  formed, 
closely  connected  with  the  amount  of  available  soil-water.  Thus  we  find 
that  in  plants  grown  under  irrigation  the  amount  of  lobing  is  very  much 
more  marked  than  in  field  plants.  We  shall  return  to  this  subject  later. 

The  first  inflorescence  is  usually  formed  early  in  the  history  of  the 
plant,  and  may  occur  in  the  first  growing  season  even  in  field  plants, 
though  this  is  exceptional  (plate  17).  This  early  flowering  in  a  shrubby 
plant  of  long  life  appears  to  reflect  its  relationship  to  herbaceous  forms, 
and  would  not  improperly  be  regarded  as  indicating  that  the  perennial 
habit  of  the  guayule  and  mariola  is,  phylogenetically,  a  recently  acquired 
character.  The  inflorescence,  which  is  a  compound  monochasium  (fig.  10) , 
is  terminal,  and  thus  ends  the  growth  of  the  chief  shoot.  In  some  in- 
stances flowering  may  not  occur  for  some  years,  and  in  this  event  if  no 
accident  befalls  the  chief  shoot  it  may  attain  a  length  of  15  cm.  or  more 
before  the  first  flower  shoot  appears  to  conclude  the  growth  of  the  chief 
axis.  In  such  a  case  the  lateral  shoots  make  but  little  growth.  Upon 
the  first  occasion  of  flowering  the  growth  of  the  branches  begins;  these 
4 


50  Guayule. 

in  turn  terminate  in  inflorescences  and,  by  ending  their  growth,  give  stim- 
ulus to  the  growth  of  branches  of  higher  orders,  each  in  its  turn.  Thus 
the  plant  becomes  profusely  branched,  and  this  habit  contributes  mate- 
rially to  the  amount  of  secretion,  which  is  proportional  to  the  number  of 
branches. 


FIG.  10. — The  inflorescence  of  the  guayule. 

THE  MATURE  PLANT. 

ROOT-SYSTEM. 

The  root-system  of  the  guayule  consists,  in  a  plant  derived  from  a 
seedling,  of  a  strong  tap-root  extending  to  a  considerable  but  undeter- 
mined depth  in  the  soil.  The  lower  end,  which  branches  more  or  less, 
draws  upon  the  water-supply  of  the  deeper  layers  of  the  soil,  especially 
in  younger  plants.  Just  below  the  surface  of  the  soil  a  number  of  strong 
lateral  roots  are  given  off,  which  in  many  instances  are  of  extraordinary 
length,  reaching  a  distance  of  150  to  200  cm.  or  more  from  the  plant 
(plate  9,  fig.  A).  These  serve  to  take  up  the  water  in  the  shallower  lay- 
ers of  the  soil,  derived  from  rains  sufficient  to  wet  the  soil  to  this  depth. 
Such  far-reaching,  shallow-placed  roots  are  characteristic  of  many  desert 
plants.  Cannon  (1909)  has  studied  and  mapped  the  root-systems  of  a 
number  of  such,  and  has  further  shown  that  competition  between  juxta- 
posed plants  may  be  eliminated  by  the  difference  in  the  type  of  root- 
system,  the  one  going  deeply,  while  the  other  is  chiefly  shallow.  The 
development  of  two  differently  placed  parts  of  the  same  root-system,  the 
one  drawing  on  the  deeper,  the  other  on  the  shallower  layers  of  the  soil,  is 
of  very  great  importance  biologically,  and  is  well  exemplified  by  the  little 
cactus  Ariocarpus  kotschubeyanus ,  which  grows  in  the  alluvial  plains  of  the 
mesa  central.  The  shallow  roots  arise  from  the  top  of  the  tap-root  and 
ascend  as  nearly  vertically  upward  as  may  be,  till  they  reach  to  within  a 
few  millimeters  of  the  surface  of  the  soil,  when  they  suddenly  take  a  hori- 


Description  of  the  Guayule.  51 

zontal  position,  and  in  this  direction  traverse  considerable  distances  from 
the  plant.  This  condition  is  closely  analogous  to  that  in  the  guayule  and 
serves  to  make  even  clearer  the  significance  of  the  arrangement  in  that 
plant  and  in  others,  in  all  of  which  the  tap-root  system,  while  quantita- 
tively inferior  both  as  regards  the  number  of  branches  and  the  amount  of 
water  absorbed  into  the  superficial  system,  may  nevertheless  be  of  a  good 
deal  of  importance  in  enabling  the  plant  to  withstand  prolonged  drought 
when  the  shallower  portions  of  the  soil  become  very  dry.  This  is  indi- 
cated by  the  readiness  with  which  retonos  arise  from  the  tap-root  after 
the  plant  and  lateral  roots  have  been  cut  away. 

RETONOS. 

From  these  shallow  lateral  roots  there  frequently  arise  new  adventi- 
tious shoots,  sometimes  singly,  sometimes  in  groups  of  two  or  more  (Lloyd, 
igoSc)  (plate  9).  They  are  locally  called  "retonos,"  though  this  term  is 
not  always  used  strictly ,  and  may  apply  to  shoots  arising  from  stem  tissues. 
It  is  the  same  word  as  "  rattoon,"  used  by  sugar-cane  planters,  but  as  this 
is  used  constantly  to  indicate  offshoots  from  the  base  of  the  stem,  it  is 
inapplicable  as  an  equivalent  of  retono.  Since  it  is  the  only  term  used  in 
Mexico  for  these  shoots  of  root  origin  and  as  our  common  English  equivalent 
is  characterized  chiefly  by  its  inelegance,  we  shall  venture  to  retain  the 
Mexican-Spanish  expression. 

Retonos  usually  arise  from  the  plant  at  a  distance  of  20  cm.  or  more. 
They  have  been  found  at  a  meter's  distance,  and  doubtless  may  occur 
still  further  away.  The  point  of  origin  may  be  above,  below,  or  at  the 
side  of  the  root.  As  growth  proceeds  the  proximal  part  of  the  root  fails 
of  further  secondary  thickening,  or  at  most  undergoes  very  little  thick- 
ening. It  ultimately  becomes  abstricted  by  decay,  apparently  induced 
by  pressure  of  the  tissues  of  the  retono,  and  quite  soon  loses  its  physio- 
logical value.  The  distal  portion,  however,  thickens  more  rapidly,  keep- 
ing pace  with  the  growing  retono,  and  takes  on  the  proportions  of  a  tap- 
root, though  it  may  always  be  distinguished  from  a  true  tap-root  by  its 
curvature  and  position  in  the  soil.  Secondary,  adventitious  roots  (fig.  n) 
later  arise  from  the  basal  portion  of  the  stem  of  the  retono,  thus  amplify- 
ing the  root-system.  A  large  root-system  thus  developed  is  shown  in  the 
central  and  largest  plant  in  plate  9,  fig.  B. 

The  author  of  this  publication  stated  as  follows  in  a  previous  paper  : 

The  formation  of  these  new  plants  in  this  manner  is  not  spasmodic  or  excep- 
tional, nor  are  they  fugitive  in  their  nature.  Under  certain  conditions  they  are  pro- 
duced in  such  numbers  as  to  entirely  overshadow  the  numbers  of  seedlings ;  and  they 
as  frequently  grow  into  maturity,  producing  a  plant  which,  if  the  origin  were  not 
known,  would  not  unlikely  be  considered  a  varietal  type,  in  point  of  habit.  The 
mature  plant  which  had  its  origin  as  a  seedling  has  a  single'trunk,  usually  10  cm., 
sometimes  20  to  30  cm.,  in  length;  the  mature  plant  produced  vegetatively  has  usu- 
ally a  very  short  trunk,  or  a  group  of  separate  ones,  more  or  less  coalesced  by  growth, 
though  marked  exceptions  may  occur  (plate  9,  fig.  C). 

The  ratio  of  the  number  of  new  plants  arising  as  seedlings  and  of  those  arising 
as  root-shoots  varies  with  the  habitat.  Both  forms  may  be  found  in  any  situation, 
but  the  retonos  are  much  more  numerous  on  stony  slopes,  often  outnumbering  the 
seedlings.  The  reverse  relation  is  seen  in  more  level  places.  Thus,  at  the  foot  of  a 
low  ridge  I  have  found  seedlings  plentiful,  as  many  as  30  in  a  square  foot  (these 
small  and  larger  ones  as  well  scattered  about  relatively  thickly).  A  zone  of  this 


52 


Guayule. 


character  could  be  traced  around  the  ridge.  Just  above  this  zone  another  could  be 
made  out  in  which  the  retonos  were  abundant  and  the  seedlings  scarce,  while 
coming  to  the  top  of  the  ridge  the  seedlings  again  outnumbered  the  retonos.  Thus 
on  that  part  of  the  slope  most  affected  by  erosion,  and  where  there  is  more  chance 
of  uncovering  the  shallow  roots,  the  retonos  are  most  abundant.  It  would  appear, 
therefore,  that  the  exposure  to  light  is  a  potent,  if  not  the  most  important,  factor 
in  inducing  budding  in  the  roots.  Yet  I  have  found  that  when  a  plant  is  removed 
by  cutting  at  the  base  so  as  to  sever  the  roots  and  leave  them  in  the  ground,  shoots 
start  from  the  root,  not  only  where  the  root  is  accidentally  exposed,  but  as  far 
back  as  the  drying  out  of  the  root  makes  it  necessary.  A  root  thus  severed  in 
January  failed  to  bud  till  June  in  consequence  of  the  lack  of  rain;  when  at  last  it 
rained,  the  buds  started  out  12  cm.  away  from  the  cut  end  and  several  centimeters 
deep  in  the  soil.  On  the  other  hand,  roots  purposely  exposed  for  a  portion  of  their 
length  and  slightly  wounded  had  failed  to  start  buds  at  the  end  of  six  months  when 
last  examined.1  So  the  case  appears  to  be  more  complicated  than  at  first  appears. 
Injury  may  be  a  factor  at  times,  but,  experimentally,  I  have  shown  that  scarring 
or  cutting  the  cortex  is  not  sufficient  to  insure  budding,  at  least  under  field  condi- 
tions, for  it  is  probable 
that  the  exposure  to  a 
low  relative  humidity  in- 
hibits the  growth  of  callus 
on  exposed  roots.  It  is 
more  probable  that  had 
roots  been  injured  and 
left  covered  with  soil, 
positive  results  would 
have  accrued. 

This  occurrence  of 
retonos  in  guayule  pre- 
sents a  very  interesting 
biological  phenomenon. 
In  a  habitat  where  the 
rainfall  is  very  meager,  so 
that  years  occur  in  which 
the  conditions  for  germi- 
nation are  prohibitive,  and  where,  moreover,  sudden  and  severe  rains  wash  the  soil 
on  the  steeper  slopes  severely  enough  to  remove  seeds  or  expose  seedlings  when 
young  so  as  to  prevent  their  further  growth,  it  will  easily  be  seen  that  the  vegetative 
method  of  reproduction  presents  certain  very  marked  advantages.  This  is  true  also 
where  the  soil  is  confined  to  the  crevices  of  the  native  rock  where  it  lies  at  or  very 
near  the  surface.  This  condition  occurs  very  frequently  in  North  Zacatecas,  where 
large  areas  will  be  seen  in  which  the  vegetation  is  confined  to  bands  of  outcropping 
rock,  where  it  occupies  the  soil  beneath  the  edge  of  a  stratum.  Where  the  relation 
of  the  strata  to  the  surface  is  such  that  flat  blocks  of  rock  support  but  a  thin  layer 
of  soil,  the  distribution  of  vegetation  is  determined  by  the  fissures.  In  the  case  of 
guayule  we  have  an  exception,  for  this  plant  may  send  out  a  shallow  lateral  root 
over  a  block  of  stone,  above  which  plants  may  start.  Very  frequently  we  find 
individuals  which  have  grown  in  this  position,  with  their  roots  straddling  the  sub- 
imposed  rock.  Such  are  almost  invariably  retonos.  Plant  i  (plate  9,  fig.  B),  was 
found  so  placed.  There  are  other  plants  which  can  compete  with  the  guayule  in 
this  regard,  such  as  the  lechuguilla  (Agave  lecheguilla) ,  which  spreads  out  by  means 
of  stolons,  and  occupies  areas  for  itself  to  the  exclusion  of  everything  else.  It  is 
clear  that  the  habit  described  is  of  no  small  importance  in  the  fight  for  foothold. 
One  can  easily  imagine,  too,  that  a  distinct  advantage  is  to  be  had  in  the  rate  of 
growth  and  the  quickness  with  which  the  ability  to  flower  abundantly  is  reached 
by  retonos.  The  rate  of  growth  is  relative  to  the  size  of  the  mother  root;  but  it  is  a 
very  common  thing  for  a  retono  to  grow  10  cm.  and  to  come  into  flower  in  two  months 
in  summer  (plate  9,  fig.  B,  10  and  n).  Seedlings,  on  the  other  hand,  flower  only 

1  Only  negative  results  were  had  as  late  as  September  1908. 


FIG.  u. — Retonos,  showing  position  of  adventitious  roots,     pr., 
proximal  portion,  and  dr.,  distal  portion  of  mother-root. 


An  exceptionally  tall  ( 1 30  cm.)  individual.     Weight  9.4  Ibs.     Caopas. 


A.  A  widely  spreading  (130  cm.)  plant  of  guayule.     Weight  10  Ib*.  9  oz. 

B.  A  large  plant  of  the  usual  habit.     Weight  8.5  Ibs.     Apizolaya. 


PLATE  12 


Description  of  the  Guayule.  53 

infrequently  before  the  third  year,  and  the  amount  of  growth  then  does  not  more  than 
equal  that  of  a  single- stemmed  retono  in  one  year.  At  the  end  of  three  years  the 
retono  makes  a  considerable  plant  (6  in  the  same  plate),  and  flowers  richly.  The 
influence  which  retonos  would  have  in  reforesting  processes,  both  by  their  own  growth 
and  by  seedlings,  can  therefore  be  well  appreciated,  and  probably  with  difficulty  over- 
estimated. Some  basis  for  judgment  in  this  regard  will  reward  a  study  of  the  accom- 
anying  photograph l  (plate  9,  fig.  B,  in  which  the  horizontal  lines  are  to  be  regarded 
as  10  cm.  apart). 

From  his  comparative  morphological  and  anatomical  studies  on  "  nor- 
mal" parts  and  those  of  individuals  ("rejets")  arising  from  root  buds, 
Dubard  (1903)  draws  the  following  conclusions: 

En  re'sume',  la  multiplication  par  bourgeons  radicaux  est  un  fait  peu  normal 
dans  le  regne  ve"ge"tal ;  elle  donne  naissance  a  des  rejets  d'organisation  infeYieure,  dans 
la  plupart  des  cas;  chez  quelques  e"speces  elle  tend  a  s'e'tablir  d'une  facon  re"guliere, 
mais  ne  devient  qu'exceptionnellement  une  sauvegarde  effective  de  1'espece. 

The  inferior  organization  of  the  retonos  studied  by  Dubard  is  always 
in  the  direction  of  an  anterior  form  "by  virtue  of  hereditary  ante- 
cedents": "les  rejets  radicaux  des  diverse  e"speces  d'un  meime  genre 
manifestent  une  convergence  qui  ne  peut  etre  fortuite." 

The  retonos  of  the  guayule  are  in  the  same  case.  The  absence  of 
medullary  and  of  cortical  canals  is  a  marked  return  to  a  more  simple 
structure,  as  is  also  the  absence  of  medullary  stereome,  in  which,  and  in 
the  absence  of  canals  in  the  medulla,  we  see  an  assumption  of  seedling 
characters.  But  the  retono  assumes  a  still  more  ancient  condition,  we 
may  believe,  in  the  loss  of  the  cortical  canals. 

Nevertheless,  the  guayule,  while  in  this  measure  conforming  to  the 
observations  made  by  Dubard,  can  not  on  any  account  be  relegated  to 
a  subnormal  category,  characterized  by  comparative  impotence  in  safe- 
guarding the  species.  The  frequently  strong  vegetative  growth ;  the  early 
maturation  of  flowers  and  seeds ;  the  already  established  root-system ;  the 
cincture  of  the  mother  root  tending  to  separate  the  retono  physiologically, 
if  not  always  structurally,  from  the  parent  plant  (fig.  n);  its  frequently 
wide  separation  from  this;  its  ability  to  gain  a  foothold  where  seedlings 
must  surely  perish;  all  these  facts  heighten  the  importance  of  the  retono, 
despite  the  relatively  small  numbers  in  which  they  are  found,  in  enabling 
the  species  to  maintain  a  foothold.  It  seems,  indeed,  not  unlikely  that 
a  further  classification  beyond  that  of  Dubard  will  be  necessary — one  for 
those  plants  in  which  the  retono  is  of  great  importance  in  this  regard. 

A  comparison  at  this  point  between  the  guayule  and  the  mariola  is 
of  special  interest,  because,  while  they  are  closely  related  species,  their 
methods  of  vegetative  reproduction  are  quite  distinct. 

In  the  first  place,  the  root-system  in  the  mariola  differs  in  that  the  laterals 
run  at  a  steeper  angle  into  the  soil.  Occasionally  retonos  are  formed,  but,  as  far 
as  my  observation  goes,  always  close  to  the  plant,  within,  say,  5  cm.  What  always 
happens,  however,  is  this:  From  the  basal  portion  of  the  stem,  where  there  are 
many  dormant  buds,  as  a  sequence  of  the  short  internodes  marking  the  slow  initial 
growth  of  the  seedling,  new,  slender  shoots  arise,  growing  to  a  height  of  30  cm., 
more  or  less,  in  two  months.  From  the  base  of  each  such  shoot  an  adventitious 
root  starts  out,  immediately  above  the  point  of  origin  of  the  shoot.  This  usually 
single  root  develops  as  a  tap-root,  and  supplies  all  the  water  for  the  daughter  shoot, 

1  F.  E.  Lloyd,  1908^. 


54  Guayule. 

which  develops  apace,  and  ultimately  becomes  an  independent  plant.  The  isthmus 
of  tissue  ^between  it  and  the  parent  plant  does  not  enlarge  much  in  any  case,  so  that 
it  is  quite  easy,  on  taking  up  a  bush  of  mariola,  to  separate  it  into  several  smaller 
plants  by  merely  breaking  off  the  functionally  independent  elements.  Thus  the 
habits  of  mariola  and  guayule  in  this  regard  are  so  different  that  one  plant,  the  former, 
remains  a  single-stemmed  shrub  of  tree-like  habit,  while  the  mariola  is  of  the  bushy 
habit.  This  marked  difference,  it  will  be  seen,  precludes  the  advisability,  though 
the  possibility  might  remain,  of  grafting  the  guayule  on  the  mariola,  a  suggestion 
which  has  been  made  on  the  assumption  that  increased  growth  might  follow  in  the 
scion.  No  economic  result  would  follow,  and  for  this  reason:  Suppose  that  we  suc- 
cessfully graft  a  piece  of  guayule  on  a  stock  of  mariola.  The  scion  grows,  but  at  the 
same  time  new  shoots  arise  from  the  base  of  the  stock  as  described,  and  their  growth 
is  so  rapid  that  in  a  month  or  two  the  guayule  shoot  is  overtopped,  and  this  ends  the 
usefulness  of  the  graft  for  economic  purposes.  We  might  very  well  make  a  graft 
for  the  purposes  of  pure  science,  but  economically  it  would  be  a  failure  (Lloyd,  1908^:) . 

Recently  it  has  been  proposed  (Escobar,  1910),  but  with  admirable 
reserve,  that  the  dissemination  of  guayule  seed  in  areas  where  only  ma- 
riola grows  may  be  attained  by  grafting  guayule  upon  it.  The  plan  ap- 
pears impracticable. 

METHOD  OF  BRANCHING. 

It  has  been  pointed  out  above  that  the  monopodial  growth  of  the 
seedling  is  brought  to  a  close  by  the  development  of  the  first  inflores- 
cence. Following  this  event,  several  of  the  uppermost  branches  make 
a  more  rapid  growth.  These  branches  in  turn  end  their  growth  each  by 
the  formation  of  an  inflorescence,  when  usually  the  two  or  three  upper- 
most buds  continue  to  lengthen.  Thus  is  produced  a  constantly  divari- 
cating system  of  stems  (plate  n,  fig.  A),  which,  if  uninjured,  results  in 
a  splendidly  symmetrical  and  closely  branched  shrub.  A  very  excep- 
tional plant,  approaching  the  ideal  form,  is  seen  in  plate  n.  Through 
failure  of  some  branches  to  develop,  irregular  forms  are  often  seen.  These 
usually  attain  a  greater  height  than  the  symmetrical  plants.  An  unusu- 
ally tall  plant  is  shown  in  plate  10,  in  which  the  irregularity  of  growth 
is  illustrated,  while  in  plate  u,  fig.  B,  a  form  more  frequently  met,  espe- 
cially in  very  rich  fields,  is  shown. 

A  comparison  with  the  mariola  is  here  pertinent,  as  there  appear 
to  be  two  types  of  guayule  in  respect  to  the  manner  of  branching,  one  of 
which  approaches  the  condition  in  mariola.  The  usual  manner  of  exten- 
sion of  the  branching  system  is  by  the  nearly  equal  growth  of  two  or  three 
branches  just  below  the  inflorescence  (plate  14,  fig.  B) .  As  will  be  seen,  the 
anatomical  distinction  between  stem  and  peduncle  is  abrupt,  and  the  dead 
and,  according  to  age,  more  or  less  disintegrated  peduncle  remains  as  a 
spur  in  the  angle  between  the  uppermost  branches.  Often  this  may  still 
be  seen  after  the  lapse  of  many  years.  No  absciss  layer  is  formed,1  and 
this  again  gives  a  suggestion  of  the  recent  departure  of  the  shrubby  type 
from  the  herbaceous  ancestor.  After  flowering,  the  dead  peduncles  re- 
main in  evidence  above  the  foliage  of  the  plant  and  form  a  conspicuous 
character.  In  the  mariola,  on  the  other  hand,  with  the  same  morphologi- 
cal basis,  a  different  habital  form  is  had.  The  stem,  as  in  guayule,  ends 
in  an  inflorescence,  is  more  slender,  and  is  beset  with  short  branches  or 

1  This  condition  is,  of  course,  common  to  many  plants,  and  is  specially  preva- 
lent among  the  Compositae. 


Description  of  the  Guayule.  55 

spurs,  which,  because  of  the  more  rapid  growth  of  the  shoot  in  mariola, 
are  more  numerously  developed.  The  transition  into  the  peduncle  is  grad- 
ual, and  not  sudden,  as  in  guayule;  this  organ  is,  therefore,  not  sharply 
delimited,  either  morphologically  or  anatomically,  and  is  leafy  and  pro- 
vided with  buds  well  up  beneath  the  inflorescence.  In  the  following  grow- 
ing season,  and  this  usually  means  in  the  following  year,  some  of  the 
short  spurs  develop  into  leafy  branches  and  in  their  turn  terminate  in 
peduncles.  These,  like  all  the  branches,  are  slender  and  tapering,  and 
their  position,  rate,  and  manner  of  growth  result  in  a  close  interweaving 
of  stems,  in  striking  contrast  with  the  guayule. 

B1OTYPES. 

Returning  to  the  subject  of  habital  types  in  the  guayule,  it  has  been 
found  that  some  plants  have  the  mariola  manner  of  growth  (plates  12 
and  13).  Instead  of  an  abrupt  termination  of  the  stem  at  the  base  of  the 
peduncle,  the  transition  is  gradual  and  the  stems  are  of  smaller  diameter 
than  in  the  usual  type.  Foliar  differences  are  to  be  noted  beyond.  The 
matter  is  possibly  of  practical  importance,  as  the  slender  branches  with 
vaguely  delimited  flower-stalks  would,  mutatis  mutandis,  contribute  to  pro- 
duce a  less  desirable  form  of  plant  from  the  point  of  view  of  production. 
A  phylogenetic  interest  also  attaches  to  it,  inasmuch  as  the  mariola  habit 
is  more  closely  comparable  to  the  herbaceous  manner  of  growth,  as  dis- 
played by  congeneric  herbaceous  species,  than  is  the  guayule  habit.  On 
this  score,  as  on  others,  the  guayule  is  the  type  more  widely  divergent 
from  the  theoretical  herbaceous  ancestor. 

These  differences  are,  indeed,  quite  fundamental,  and  may  be  traced 
back  to  the  earliest  seedling  stages  (plate  13).  The  clearness  of  the  dis- 
tinctions is  such  as  to  indicate  that  we  are  dealing  with  a  field  mutant, 
and  the  differences  in  the  structure  of  the  awns  (pappus)  would  seem  suffi- 
cient ground,  in  the  light  of  the  taxonomy  of  the  genus,  to  warrant  us  in 
regarding  the  broad-leafed  type  as  a  distinct  species.  The  two  forms, 
P.  argentatum  proper  and  this  closely  related  form,  be  it  a  well-marked 
species  or  a  type  of  less  taxonomic  evaluation,  are  remarkably  distinct,' 
and  call  to  mind  many  similar  juxtapositions  of  closely  related  species, 
recognized  as  such,  known  to  occur  among  plants,  but  not  yet  properly 
appreciated  as  evidence  in  the  discussion  of  isolation  (Lloyd,  19056). 

Another  difference  in  the  habit — though  not  correlated,  it  appears, 
with  the  manner  of  development  of  the  inflorescence — is  seen  in  what 
may  be  termed  straight  and  crooked  limbed  forms.  The  one  is  clean- 
cut  and  smooth-limbed,  each  span  of  growth  being  nearly  straight;  the 
other  is  rougher  barked,  the  more  slender  limbs  showing  marked  curva- 
tures. The  former  is  the  more  rapidly  growing  type,  suggesting  differ- 
ences in  the  available  water-supply.  One  frequently  finds  examples  of 
very  marked  growth  differences  in  field  plants,  such  as  are  shown  in 
plate  9,  fig.  A,  of  which  the  right-hand  plant  grew  in  a  shallow  rock  crev- 
ice and  was  unable  to  develop  a  competent  root-system.  The  annual 
accretions  of  growth  in  this  plant  were  very  short,  not  exceeding  a  centi- 
meter, and  this  resulted  in  the  production  of  a  very  dense,  much-branched 
mass  of  limbs,  as  seen  in  plate  9,  fig.  A,  on  the  extreme  right.  This  and 


56  Guayule. 

the  left-hand  plant  in  the  same  figure  show  extremes  of  rate  of  growth, 
somewhere  between  which  lies  the  average,  which  it  is  desirable  to  know 
in  estimating  the  rate  of  reproduction. 

A  still  further  difference  in  habit,  which  is  not  very  readily  distin- 
guished from  the  foregoing  at  first  glance,  is  one  recognized  by  persons 
engaged  in  the  gathering  of  the  shrub,  who  designate  the  two  types  in 
question  "macho  "or  male  and"hembra"or  female.  The  differences,  which 
are  shown  in  plate  14,  fig.  B,  were  pointed  out  to  me  by  Don  Jose'  Herrera, 
a  gentleman  who  has  had  a  great  deal  of  practical  experience  in  collect- 
ing shrub.  "  Macho  "  guayule  has  fewer  branches,  and  they  have  a  larger 
diameter  than  those  of  the  "hembra,"in  which  the  branches  are  much 
more  numerous.  These  terms  are  not  here  used  in  the  sense  spoken  of  on 
page  4,  to  distinguish  guayule  from  mariola,  which  latter  is  sometimes 
called  "hembra  de  guayule,"  but  merely  to  designate  the  plant  with  the 
stronger  and  therefore  "macho  "  habit  and  that  with  the  weaker  or  "  hem- 
bra"  habit.  These  adjectives  are  used  analogously  with  respect  to  other 
plants  showing  similar  differences.  "  Hembra  "  guayule  makes  greater  bulk 
when  made  up  into  bales,  and  for  this  reason  those  who  gather  shrub  pre- 
fer to  take  it  if  they  are  being  paid  at  a  rate  per  bale.  Whether  the  dif- 
ferences are  biotypic  or  are  due  merely  to  environmental  conditions  can 
not  be  said;  nor  whether  there  are  other  correlated  differences,  as  in  the 
amount  of  rubber  secreted,  though  such  are  variously  claimed  to  obtain. 
There  appears  to  be  a  stronger  tendency  in  the "  hembra  "for  the  branches 
to  run  out  into  inflorescences,  entailing  a  greater  amount  of  dying  back 
at  the  close  of  each  growing-season,  and  thus  it  may  turn  out  that  these 
differences  are  essentially  the  same  as  those  mentioned  previously. 

Finally,  many  guayule  gatherers  and  others  think  to  recognize  dif- 
ferent kinds  as  to  color-characters,  either  of  the  bark  or  of  the  leaves.  In 
Durango  white  guayule  ("bianco")  is  distinguished  from  dark  or  "  prieto," 
though  no  other  characters  could  be  pointed  out  to  separate  the  two 
kinds.  Indeed,  when  a  branch  was  exposed  to  view  in  one  position,  so  that 
the  under  surface  of  the  twigs  was  seen,  it  was  pronounced  "prieto," 
and  when  the  upper  surface  of  the  same  branch  was  later  shown  it  was 
called  "bianco."  This  color  difference,  as  between  the  upper  and  lower 
surfaces  of  the  branches,  is  quite  constant. 

"Blanco"  and  "ceniso"  or  ashy  guayule  are  maintained  to  be  dif- 
ferent also,  though  the  same  difficulty  of  seizing  upon  other  than  mere 
color  differences  obtains.  So  far  as  I  could  determine,  "ceniso"  guayule 
was  shrub  which  had  been  exposed  to  severer  drought,  in  consequence 
of  shallower  soil  in  exposed  positions,  as  on  benches,  and  in  which  the 
leaves  had  therefore  dried  to  a  dirty-yellowish  color.  Prolonged  study 
might ,  however,  discover  that  some  of  these  differences  are  constant  and 
racial,  and  the  matter  therefore  deserves  more  consideration. 

SIZE. 

The  question  is  often  asked,  especially  by  persons  interested  from 
the  business  point  of  view,  as  to  the  size  which  the  guayule  attains.  It 
may  at  once  be  said  that  anything  like  the  maximum  size  is  a  matter, 
or  will  be  shortly,  of  academic  rather  than  economic  interest.  Once  the 


LLOYD 


A.  An  irrigated  plant,  from  a  small  stock,  at  the  height  of  flowering. 

B.  "Embra"  (on  the  left)  and  "Macho"  (on  the  right)  guayule. 


Description  of  the  Guayule.  57 

virgin  guayule  has  been  removed,  big  plants  will  no  more  be  seen.  The 
largest  plants  which  have  been  reported  weighed  in  the  neighborhood  of 
10  kilograms.  Overseers  of  field  experience  insist  that  they  have  seen 
and  weighed  such.  Endlich  (1905)  quotes  Marse  as  having  seen  a  plant 
weighing  6.5  kilos;  but  a  plant  weighing  over  5  kilos  is  exceptional.  Of 
three  large  plants  which  are  illustrated  in  this  paper,  that  in  plate  12, 
fig.  A,  weighed  10.56  pounds  (fresh  weight),  was  75  cm.  tall,  and  125  cm. 
wide.  Plants  over  a  meter  in  height  are  seldom  met  with,  and  are  nearly 
always  more  or  less  stag-headed  end  moribund  (plate  10).  They  have 
usually  lost  a  good  many  limbs,  and  for  many  years  have  not  been  mak- 
ing any  net  gain  in  weight.  Endlich  places  the  average  weight  of  virgin 
guayule  at  500  or  600  grams.  As  will  develop  in  the  discussion  in  the 
following  chapter,  plants  of  this  size,  which  would  be  40  to  50  cm.  or 
more  tall,  will  in  the  future  be  considered  large  plants. 

SURFACE  CHARACTERS  OF  THE  STEM  AND  METHOD 
OF  DETERMINING  AGE. 

The  importance  and  difficulty  of  determining  accurately  the  age  of  a 
particular  guayule  plant  has  prompted  careful  study  of  the  appearance  of 
the  surface  of  the  stem  at  various  ages  (plate  14,  fig.  B).  This  appearance 
is  due  to  (i)  the  primary  superficial  characters  (epidermis,  leaf-scars)  and 
(2)  the  succeeding  secondary  cork.  Secondary  changes  in  the  cork  are 
produced  by  weathering.  As  marks  also  aiding  in  the  determination  of 
age  may  be  mentioned  the  dead  but  persistent  peduncles  and  the  number 
of  divarications  of  the  stem,  as  related  to  the  formation  of  inflorescences. 
Data  relating  to  the  rate  of  growth  of  seedlings,  the  marks  of  which  are 
usually  quite  obliterated  in  plants  taller  than  10  or  15  cm.,  must  also  be 
considered. 

FIELD  PLANTS. 

Let  us  suppose  that  we  are  examining  a  plant  at  the  close  of  the 
growing  season  of,  say,  1908.  The  characters  seen  in  the  accretions  for 
the  years  mentioned  will  be  as  follows : 

1908.  Leaves  still  adherent.  The  epidermis  is  intact  and  densely 
clothed  with  appressed  T-shaped  hairs,  producing  the  greenish-gray  color 
uniform  with  the  leaves.  If  the  length  of  the  year's  growth  is  exceptional, 
say  above  10  cm.,  the  basal  part  may  show  slight  longitudinal  fissures. 
Diameter  at  base  3  mm.  or  less,  rarely  more. 

1907.  Epidermis  still  adherent,  but  more  or  less  fissured,  showing 
yellow  cork.  The  hairs  have  been  partially  removed  by  attrition  and 
withering,  but  most  of  them  remain,  preserving  a  gray  color.  Epidermis 
light  brown.  Leafless,  but  scars  present.  Often  with  short  spurs,  or  un- 
developed branches  with  each  a  few  leaves.  Diameter  usually  between  3 
and  4  mm. 

1906.  Color  gray,  slightly  slaty  brownish,  generally  fissured,  the  fis- 
sures shallow,  disclosing  a  gray-colored  cork  (weathered),  with  small  areas 
of  epidermis  remaining  between.  Diameter  about  5  mm. 

1905.  The  growth  for  this  and  earlier  years  is  dark  gray,  becoming 
darker  with  age.  The  fissures  are  shallow,  becoming  deep  only  with  an 
age  of  over  10  years.  The  fissuring  is  deeper,  and  lenticels  are  more  abun- 


58  Guayule. 

dant  on  the  lower  surface  of  stems  which  are  not  in  a  vertical  position. 
This  is  because  of  the  thicker  development  of  bark  on  this  side.  On  old 
stems  the  fissures  attain  a  depth  of  a  few  millimeters  and  become  long.  On 
very  old  stems  the  base  may  become  transversely  fissured  also  (plate  10). 

In  using  the  above  marks  as  a  means  of  judging  the  age  of  a  plant, 
one  may  with  considerable  accuracy  judge  of  the  amount  of  growth  for 
3  or  4  years,  and  the  average  of  these  will  come  very  near  to  the  truth. 
Some  difficulty  may  be  experienced  as  the  result  of  reduplicated  growth 
in  one  year  confusing  the  evidence  offered  by  the  leaf-scars,  which  are 
crowded  fairly  closely  in  the  region  where  the  internodes  of  the  winter 
buds  occur.  These  are  of  the  tropical  type,  there  being  no  specialized 
scale-leaves,  and  consist  merely  of  a  few  terminal  leaves  of  small  size 
which  persist  till  the  following  season  of  growth. 

The  natural  wounding  which  results  in  fissures,  especially  as  the  stem 
grows  older,  as  well  as  the  accidental  wounding  which  frequently  occurs, 
usually  sets  free  more  or  less  of  the  resin,1  of  which  large  amounts  are  found 
in  the  cortex,  as  in  the  pith.  The  escaping  resin  collects  as  drops  on  the 
wound  and,  as  it  increases  in  amount,  falls  on  the  ground.  Under  every 
guayule  plant  of  any  size,  therefore,  a  good  deal  of  resin  in  the  form  of 
limpid  masses  of  irregular  size  may  be  found.  Should  it  turn  out  that  the 
resin  is  of  particular  value  (Chute,  1 909) ,  as  for  a  special  varnish,  consider- 
able amounts  could  be  collected  by  peons. 

IRRIGATED  PLANTS. 

In  irrigated  plants  secondary  thickening  begins  within  a  short  dis- 
tance (5  to  15  mm.)  of  the  growing-point,  and  proceeds  at  a  rapid  rate. 
The  fissures  are  very  long  and  straight,  and  long  patches  of  epidermis 
are  left  which  may  be  still  visible  30  to  40  cm.  from  the  apex.  The  color 
for  two  years  remains  a  clean,  pale  yellow,  modified  by  the  gray  of  the 
adherent  hairs  wherever  patches  of  epidermis  remain  (plate  21,  fig.  A). 
The  diameter,  which  remains  nearly  the  same  throughout  the  length  of 
a  year's  growth  in  a  field  plant,  making  the  growth  cylindrical,  increases 
rapidly  in  irrigated  plants,  so  that  the  basal  diameter  may  be  three  times 
that  of  the  tip  in  the  first  year  and  eight  times  at  the  end  of  the  second 
year.  The  early  fissuring  and  the  coloring  are  correlated  with  this  rapid 
secondary  thickening. 

THE  LEAVES. 

The  leaves  of  seedlings  have  already  been  described.  In  the  adult 
plant  the  form  of  the  leaf  varies  according  to  the  amount  of  water  avail- 
able and  its  position  on  the  twig.  In  general  the  water-factor  determines 
the  amount  of  lobing.  This  is  apparent  in  field  plants  as  well  as  in  those 
grown  under  irrigation,  and  the  relation  is  made  manifest,  in  field  forms 
especially,  in  the  sequence  of  leaf -form  seen  during  the  growing  and  the 
.subsequent  resting  period,  consequent  on  drought  and  cooler  tempera- 
tures. The  guayule  may  be  called  semi-deciduous,  as  it  sheds  a  part  of 
the  leaves  only,  namely,  those  which  are  produced  between  the  more  elon- 

1  Loss  of  resin  by  secondary  thickening  is  for  the  most  part  prevented  by 
plugging  of  the  resin-canals  (Chapter  V). 


Description  of  the  Guayule.  59 

gated  internodes.  Those  which  are  still  crowded  together  in  the  terminal 
bud-cluster  remain  and  form  the  basal  leaves  of  the  subsequent  season's 
growth.  These  leaves  are  the  last  to  be  developed,  that  is,  at  the  close  of 
the  growing-season.  Since  the  length  of  the  season  is  determined  chiefly 
by  the  decrease  of  soil-water,  the  shape  of  these  last-formed  leaves  seems 
to  be  conditioned  by  this  circumstance.  This  is  evidenced  by  the  fact 
that  irrigated  plants,  to  which  water  is  available,  continue  to  form  lobed 
leaves  (plate  21),  and  even  those  which  compose  the  terminal  bud  are,  in 
some  plants,  as  deeply  lobed  as  the  rest. 

The  winter  leaves,  as  we  may  call  those  which  persist  in  the  terminal 
bud,  are  from  i  to  3  cm.  long  by  3  to  7  mm.  broad,  elongate-ovate,  taper- 
ing into  the  petiole,  entire,  or  with  one  or  two  very  much  reduced  teeth, 
acute.  The  summer  leaves  are  6  to  7  cm.  long  by  2  to  2.5  broad  when  full- 
sized,  and  are  deeply  lobed  midway  the  length  of  the  blade.  A  large 
amount  of  variation  is  met  with  in  these  leaves,  however,  the  form  depart- 
ing from  the  proportion  given  to  a  long,  slender,  merely  toothed  leaf,  7  by 
0.7  cm.  The  summer  leaves  persist,  in  field  plants,  till  December  or  later, 
at  which  time  they  begin  to  fall.  By  the  middle  of  February  all  the  leaves 
excepting  the  terminal  bud-leaves  have  fallen,  leaving  the  gray  twigs  bare, 
each  surmounted  by  its  leaf-cluster  (plate  14,  fig.  B).  Leaf-fall  appears 
to  be  a  function  of  drought  rather  than  temperature.  Long  before  falling 
the  leaves  show  marked  shriveling  and  curling,  and  fall  away  as  much  by 
drying  off  as  by  the  action  of  an  absciss  layer  (see  Chapter  V) ,  which  is 
imperfectly  formed.  In  irrigated  plants  leaf- fall  is  much  less  prompt, 
proceeding  from  the  base  of  the  previous  season's  growth  upward,  the  pro- 
cess not  being  completed  much  before  the  following  April. 

THE  INFLORESCENCE  AND  THE  FLOWERING-PERIOD. 

The  growth-period  of  guayule  is  indeterminate  and  is  largely  a  re- 
sponse to  moisture  conditions,  within  certain  relatively  wide  limits  (Chap- 
ter IV).  Similarly,  the  formation  of  flower-buds  occurs  as  a  function  of 
this  growth  and  is  not  related  to  temperature  or  other  seasonal  conditions. 
Thus,  if  the  growth  is  small  in  amount  only  that  flower-bud  which  hap- 
pens to  be  ready  to  expand  will  be  developed.  If  the  amount  is  great  a 
second  or  even  third  series  of  flower-buds  may  be  developed  and  come  into 
fruition,  though  it  is  seldom  that  more  than  two  series  mature  in  one  year. 
When  the  summer  rains  commence  the  resting  buds,  with  their  frequently 
inclosed  and  partially  developed  flower-buds,  soon  begin  to  grow,  and 
forthwith  the  first  series  of  flowers  is  developed. 

According  to  my  data  for  1 908  there  was  practically  no  growth  at  all 
till  somewhat  later  than  May  22.  By  June  9,  in  more  favorable  situations, 
as  in  arroyo  beds,  plants  were  found  in  full  flower,  and  by  about  the  mid- 
dle of  the  month  flowering  was  well  started  on  the  ridges  of  the  foot-slopes 
and  in  the  hills.  In  certain  unfavorable  localities,  e.g.,  on  low  ridges  in 
the  plains  west  of  Cedros,  the  peduncles  had  attained,  by  July  22,  only 
half  their  normal  growth.  The  flowering  of  the  hill  plants  continued  for 
a  month,  seed  ripening  and  new  flowers  coming  on,  when,  by  the  middle 
of  August,  the  vigorous  flowering-period  was  entirely  closed.  By  Sep- 
tember 9,  up  to  which  time  there  was  more  or  less  spasmodic  flowering, 


60  Guayule. 

the  period  was  at  an  end.  This  does  not  mean,  however,  that  fresh  flower- 
buds  were  not  available  and  ready  to  develop,  but  that  the  water-supply 
was  insufficient  to  support  the  heavy  foliage  and  to  enable  the  full  devel- 
opment of  the  flowers  as  well. 

The  end  of  the  flowering  season  is  shown  as  much  by  the  abortion  of 
the  immature  capitula  as  by  any  other  behavior.  This  is  but  the  extreme 
expression  of  a  very  general  phenomenon,  that  of  the  unequal  development 
of  the  inflorescence  in  adjoining  situations.  "  When  water  is  abundant 
the  inflorescence  is  widely  spreading,  the  result  of  the  development  of 
the  pedicels  (fig.  10),  while  where  the  water-supply  is  meager,  but  not 
insufficient  for  the  development  of  the  flowers,  the  pedicels  may  remain 
very  short  and  thus  produce  a  crowded  mass  of  capitula.  This  condition 
is  usually  met  with  in  the  field  (plate  2) ,  and  between  this  and  complete 
abortion  of  the  flowers  every  degree  of  failure  to  flower  is  seen,  the  result 
of  reduced  water-supply. 

While  the  grand  flowering-period  falls  normally  in  the  summer,  the 
exigencies  of  rainfall  may  so  modify  the  rhythm  of  the  plant  that  it  will 
occur  in,  possibly,  any  month  of  the  year.  Under  irrigation  flowering 
starts  in  March,1  and  there  is  sustained  a  profusion  of  flowers  through 
April  (plate  14,  fig.  A)  and  May.  It  then  dwindles,  a  second  period  of 
low  maximum  occurring  in  August,  to  be  continued  irregularly  and  with 
less  perfectly  developed  flowers  into  November.  In  the  field  abundant 
flowers  were  observed  in  October  in  Durango  (Hacienda  de  los  Sombre- 
retillos)  and  in  Sierra  Ramirez,  Zacatecas.  Up  to  this  time  of  the  same 
year  (1907)  no  flowers  had  been  produced  in  the  Sierra  Mojada  on  the 
Hacienda  Santa  Inez,  in  Durango,  where  the  guayule  plants,  forming  an 
almost  pure  culture,  were  in  a  shriveled  condition  for  lack  of  water. 

Under  favorable  conditions  the  development  of  the  inflorescence  takes 
about  two  weeks.  The  flowers  emit  a  delightful  fragrance  which  attracts 
many  small  insects.  Among  these  visitors  mosquitoes  were  observed, 
extracting  the  nectar  from  the  ray-flowers. 

THE  PRODUCTION  OF  SEED. 

Though  the  maximum  number  of  seeds  which  may  be  produced  by 
each  capitulum  is  only  5,  the  total  number  yielded  by  a  moderate-sized 
plant  may  amount  to  many  thousands.  The  percentage  of  viable  seed, 
however,  runs  small.  In  a  field-plant  with  well-developed  heads  less  than 
5  per  cent  of  well-filled  achenes  were  found.  In  other  plants  as  high  as 
2  5  per  cent  were  found  filled.  In  irrigated  plants  the  percentage  rises  con- 
siderably higher,  namely,  to  about  35  per  cent.  When  the  achenes  are 
fully  ripe  the  bracts  become  brown  in  color  and  fall  away  from  the  pedi- 
cels quite  easily.  The  collection  of  seed  (Chapte^  IX),  which  must  be 
done  by  hand  if  at  all,  should  begin  to  give  the  best  results  at  the  close  of 
the  first  period  of  flowering.  Properly  done,  the  flowers  are  stripped  from 
the  peduncle,  which  need  not  be  removed  from  the  plant.  The  nature  of 
the  "seed"  has  already  been  discussed. 

1  In  1909  flowering  did  not  begin  in  these  plants  before  the  middle  of  April. 
Inquiry  developed  that  they  had  not  been  irrigated  freely,  if  at  all,  though  of 
course  the  soil  was  much  better  supplied  with  moisture  than  that  of  the  field. 


5  cm, 


CHAPTER  IV. 
REPRODUCTION. 

METHODS  OF  REPRODUCTION. 

It  is  the  purpose  of  the  present  chapter  to  compare  the  two  methods 
of  reproduction,  sexual  and  vegetative,  with  reference  to  final  efficiency 
in  reproducing  the  species.  It  need  scarcely  be  said  that,  in  speaking  of 
sexual  reproduction,  we  are  using  the  term  to  indicate  the  origin  of  the 
seed.  It  will  be  at  once  accepted  that  accurate  knowledge  of  the  topic  here 
to  be  considered  is  of  vital  importance  in  deriving  estimates  of  the  rate 
at  which  guayule  fields  may  be  expected  to  produce  a  crop  of  that  plant. 

From  what  has  been  said  in  the  foregoing  chapter  it  will  be  seen 
that,  taking  different  kinds  of  habitats  into  account,  an  average  rate  of 
reproduction  will  be  maintained  by  means  of  the  retofio  and  seed  methods 
combined.  The  relative  efficiency  of  the  two  methods  depends  upon 
widely  different  considerations,  and  these,  as  we  shall  now  see,  have  rela- 
tion to  numbers  of  individuals,  rate  of  growth,  and  the  time  of  the  year  at 
which  they  begin  this  growth. 

RETONOS,  NORMAL  AND  INDUCED. 

We  may  speak  of  two  kinds  of  retonos,  normal  and  induced.  By 
normal  we  mean  those  which  arise  spontaneously  upon  the  lateral, 
superficially  placed,  horizontal  roots,  remaining  for  some  time  attached 
to  the  plants  from  which  they  spring  (plate  9).  Induced  retonos  (plate 
15)  will  then  be  those  which  arise  as  the  result  of  mutilation,  that  is, 
from  roots,  primary  or  of  a  higher  order,  after  the  plant  has  been  cut 
away.  This  is  done  on  a  number  of  haciendas  in  the  harvesting  of  the 
shrub,  whereas  the  plants  used  to  be,  and  by  many  still  are,  pulled  up  by 
the  roots.  This  pulling  results,  of  course,  in  breaking  away  many  of  the 
roots,  but  the  chief  portion  of  the  tap-root  is  removed,  as  also  are  consid- 
erable lengths  of  the  other  roots.  As  we  shall  see,  the  difference  in  effect 
upon  reproduction  is  merely  quantitative,  as  in  both  cases  retonos  may 
arise,  but  in  very  different  numbers.  In  order  to  test  this  with  as  great 
accuracy  as  possible,  quadrats  of  100  square  meters  were  cleared  of  the 
guayule  both  by  the  cutting  and  pulling  methods,  and  the  results  were 
noted.  These,  for  the  quadrats  observed,  afford  accurate  data,  which 
must  be  understood  as  of  indicatory  value  only.  It  may  well  be  believed 
that  different  meteorological  conditions  would  have  modified  the  results 
very  considerably.  Thus,  if  the  experiments  had  been  started  just  at 
the  beginning  of  the  summer  rainy  season  more  hopeful  results  might 
have  been  had,  but  we  shall  see  that  cutting  at  this  time  is  for  other 
reasons  an  unfortunate  practice,  and  the  evil  resulting  would  offset  the 
value  of  the  data  thus  obtained.  It  is  well,  therefore,  for  economic  rea- 
sons, that  the  data  collated  shall  be  well  within  bounds.  In  addition 
to  experimentally  obtained  data,  others  derived  from  observation  are 
given,  and  have  already  been  discussed  in  part  in  Chapter  III. 

61 


62 


Guayule. 


GENERAL  OBSERVATIONS. 

It  is  generally  believed  that,  after  a  field  has  been  harvested  of  its 
guayule,  it  will  reproduce  itself  in  a  short  period  of  years,  the  length  of 
which  is  a  matter  of  opinion.  Estimates  on  this  point  vary  from  5  to 
10  years.1  As,  however,  this  difference  in  length  of  reproductive  period, 
which  we  may  call  the  period  of  rotation,  involves  so  large  an  error  in 
returns  on  investment,  an  effort  to  get  at  the  facts  is  eminently  justified. 
From  the  botanical  point  of  view,  the  rate  of  reproduction  and  of  growth 
of  desert  plants  has  been  so  little  studied  that  data  bearing  on  these 
questions  are  of  great  importance,  especially  as  the  eye  of  civilization 
is  being  turned  on  the  desert  as  a  field  in  which  must  be  developed  the 
natural  resources  peculiar  to  it. 

NORMAL  RETOXOS. 

The  number  of  plants  which  arise  as  retonos  within  a  given  area  is, 
with  probably  few  exceptions,  small. 

TABLE  20. — Comparative  numbers  of  seedlings  and  retonos  in  given  areas. 


Local 

ty. 

Number  of  small  plants 
(below  8  oz.). 

Station. 

Quadrat. 

Seedlings. 

Retonos. 

8.  ... 

2 

36 

4 

8  

I 

14 

2 

9  

I 

86 

4 

9  

2 

5 

I 

10.  .  .  . 

9° 

8 

ii  

232 

0 

12  

200 

0 

These  numbers  are  accurate  as  far  as  they  go,  but  they  do  not  tell 
what  proportion  of  all  the  plants  of  the  quadrats  mentioned  arose  as 
retonos.  In  the  vicinity  of  Station  2  plants  of  this  sort  could  easily  be 
found,  and  all  but  one  of  those  in  plate  9,  fig.  B,  were  obtained  in  a  restricted 
area  nearby,  especially  on  the  steeper  slopes.  But  for  all  this,  the  total 
numbers  of  plants  which  have  arisen  as  seedlings,  taking  all  the  areas 
into  consideration,  must  far  outnumber  retono  plants.  On  irrigated  plants 
2  years  old,  some  150  in  number,  not  a  single  retono  was  observed,  a 
fact  which  may  perhaps  be  correlated  with  the  weaker  development  of 
shallow  lateral  roots  in  such  plants.  Only  one  instance  (plate  46,  fig.  B)  of 
a  retono  starting  under  irrigation  has  come  to  my  notice.  Numerous  ad- 
ventitious buds  were  distributed  on  the  mother-root,  evidently  having 
started  after  the  plant  was  pollarded.  This  was  done,  not  at  the  time  of 
transplanting,  but  some  time  later,  when  it  was  discovered  that  the  plant 
was  not  responding.  The  importance  of  normal  retonos,  therefore,  is  not 
to  be  seen  in  the  numbers  but  in  other  qualities  (Chapter  III). 

1  At  the  present  writing  we  read  in  a  recent  number  of  the  India  Rubber  World 
(March  1909),  that  a  new  crop  of  guayule  may  be  expected  in  "  a  few  years."  We 
may  suppose  that  heavily  interested  investors  have  obtained  accurate  information 
upon  which  they  base  their  operations,  but  none,  so  far  as  we  are  aware,  have  been 
given  publicity. 


Reproduction. 


63 


Normal  retonos  usually  begin  their  growth  with  the  oncoming  of 
rain,  especially  in  spring  and  early  summer.  In  this  regard  they  act 
merely  as  expressions  of  growth  and  have  no  special  peculiarities.  Start- 
ing as  they  do  from  the  shallow-lying  roots,  they  make  an  etiolated  growth 
of  a  few  centimeters  before  emerging  from  the  soil.  Their  rate  of  growth 
depends  upon  the  size  of  the  root  from  which  they  spring  and  the  num- 
bers arising  at  one  point.  If  the  root  is  slender  growth  is  relatively  slow, 
and  subsequently  depends  on  the  rate  of  secondary  growth  of  its  distal 
portion;  if  large,  the  retofio  grows  rapidly  and  may  in  a  month  or  two 
attain  a  height  of  10  or  1 5  cm.,  a  rate  scarcely  to  be  met  with  in  the  case  of 
seedlings.  A  notion  of  the  rate  of  growth  may  be  had  from  the  follow- 
ing table  of  measurements,  based  upon  the  specimens  in  plate  9,  fig.  B, 
the  numbers  referring  to  those  similarly  numbered  in  the  figure. 

TABLE  21. — Size,  age  and  weight  of  plants  which  arose  as  retonos 
(referring  to  plants  in  plate  9,  fig.  B.) 


Plant. 

No.  of 
stems 
at  base. 

Height  of 
stem. 

Diameter  of 
stems. 

Weight,  fresh. 

Age. 

cm. 

mm. 

Ibs.       oz. 

yrs.          mos. 

[23] 

I  .  . 

4 

43 

21 

24 

'2         8 

8  to  9 

I  i6j 

2   .  . 

I 

28 

20 

8 

8 

3  •  • 

7 

20 

4  .  5  to  8 

3-5 

4 

4  •  • 

5 

16 

5  to  8  .  5 

6 

4 

3 

14 

5  to  8 

2-875 

3 

6  .  . 

3 

20 

7.5  to  10 

3-25 

3 

7  •  • 

i 

18 

i7-3 

3 

4 

8  .. 

4 

14 

4 

2o.s6 

2 

9  •  • 

2 

13 

3 

2 

3IO.. 

4 

10 

i  •  5  to  2  .  5 

4 

4n  .  . 

3 

6 

2-5 

2  to  3 

JDry  weight  i  Ib.  5  oz.               'Induced  by  cutting  away  the  plant,  January  1908. 
2  Dry  weight.                                4Grew  in  season  of  1908. 

It  is  at  once  apparent  that,  as  compared  with  the  rate  of  growth  of 
seedlings,  that  of  rotonos  is  much  more  rapid.  It  takes  at  least  15  years 
to  produce  a  plant  of  2  pounds  weight  from  the  seed.  Plant  No.  i ,  in  the 
above  table,  made  its  weight  in  certainly  not  more  than  9  years,  possibly 
in  8.  This  is  brought  about  by  (i)  the  more  numerous  stems  arising  from 
the  base  and  (2)  the  more  rapid  elongation  of  the  stems,  due  to  the  ad- 
vantage had  in  the  already  established  root -system.  Table  20  affords 
comparative  data  as  between  seedlings  and  retonos.  Incidental  advan- 
tages accruing  from  this  purely  vegetative  method  of  reproduction  are  (i) 
relative  certainty  of  success  because  of  the  previous  establishment  of  the 
parent  plant,  with  relative  independence  of  an  initial  good  season  in  order 
to  start,  and  (2)  the  rapidity  with  which  the  plants  arrive  at  a  condition 
to  flower  abundantly ;  e.g.,  plant  No.  1 1 ,  a  few  months  old,  produced  fully 
100  seeds.  These,  in  a  desert  especially,  are  no  mean  advantages.  Thus, 
they  would  enable  a  single  guayule  plant  to  compete  with  such  a  plant 
as  the  lechuguilla,  assuming  that  it  had  so  fully  occupied  the  ground  that 


64 


Guayule. 


seeds  could  not  get  started,  by  maintaining  a  foothold  till  the  dying  off  of 
lechuguilla  plants,  say  as  the  result  of  flowering,  allowed  seedlings  once 
more  to  take  hold. 

INDUCED  RETONOS. 

In  order  to  determine  the  number  of  retonos  formed  after  pulling  up 
(usually  called  "cortando")  and  after  cutting  away  guayule  plants,  the 
following  experiments  were  made: 

Experiment  115. — Station  2,  quadrat  4.  Jan.  6, 1908.  250  plants,  all 
under  40  cm.  in  height,  were  pulled  up  by  hand,  leaving  in  the  ground  only 
such  roots  as  were  broken  off  by  chance.  Feb.  18,  no  growth;  Mar.  29, 
no  growth;  Apr.  28,5  roots  produced  retonos ;  July  28,9  clumps  of  shoots 
from  as  many  roots  started.  Sept.  12,  none  additional.  Apr.  3,  1909,  6 
additional  roots  had  started. 

The  following  measurements  were  made  of  dried  material  collected 
on  April  3, 1909  : 

TABLE  22. — Station  2,  quadrat  4.    Induced  retonos. 


No.  of 

stems  on 
each  root. 

Height  of  stem. 

Diameter  of 
stem  at  base. 

No.  of 
stems  on 
1  each    root. 

Height  of  stem 

j     Diameter  of 
stem  at  base. 

cm. 

mm 

cm 

mm. 

13  .5 

7,    8 

I 

6' 

3 

13 

3,  1° 

5 

4  to  7 

3  to  4 

IS 

7.    9 

6 

3  to  4  .  5 

to  3 

9 

10 

3 

4  to  8 

to  4 

10  •  5 

8 

3 

4  •  5  to  7 

to  4 

6 

6 

3  to  4 

25 

i  to  7 

to  4 

i 

8 

5 

* 

9,    10 

-  4-5 

10 

2  to  7 

2  to  4 

1                    ; 

The  average  amount  of  growth  in  stem -length  was  8  cm. ;  in  diameter 
4.4  mm.  All  of  the  new  growths  produced  flowers,  and  were  in  normal 
condition  when  examined  at  the  close  of  a  long  drought  period.  One  of 
them  is  shown  in  plate  15,  fig.  A. 

Experiment  114. — Station  2,  quadrat  3,  Jan.  6,  1908.  Of  338  plants, 
all  but  88  small  ones  (i.e.,  250  plants)  were  cut  off  with  a  "talacho" 
from  i  to  5  cm.  below  the  surface  of  the  ground.  No  growth  till  after  Mar. 
29.  Apr.  28,  1908,  40  clumps  of  new  shoots  well  started,  each  clump  of 
2  to  6  shoots.  Stems  4  to  6  cm.  long,  with  leaves  of  the  same  length.  The 
severed  roots  died  back  about  2  cm.  before  the  new  shoots  started.  Depth 
of  soil  at  which  the  shoots  started,  2.4  cm.  July  28,  59  clumps  of  new 
shoots.  Sept.  1 2,  none  additional.  Length  of  longest  stems,  10  cm.  On 
Apr.  3,  1909,  6  clumps  were  removed  and  measured,  the  data  from  which 
are  given  in  table  23. 

TABLE  23. — Station  2,  quadrat  3.    Induced  retonos. 


No.  of  stems 
in  clump. 

Length. 

Diameter. 

No.  of  stems 
in  clump. 

Length. 

Diameter. 

cm. 

mm. 

cm. 

mm. 

2 

13 

5 

4 

6 

i  to  5 

12 

7 

i  to  4 

I 

6 

'3.5 

2 

9 

i,    4 

I 

10 

8 

Ave. 

8-3 

5 

Ave. 

8-3 

5 

'From  a  tap-root  only  4.5  mm.  in  diameter. 


Reproduction. 


Close  by  this  quadrat  a  retofio  (plate  15,  fig.  B)  was  collected,  which 
had  sprung  from  a  lateral  root  5  mm.  in  diameter.  The  chief  shoot  had 
10  branches.  Total  height,  8.5  cm.;  diameter  at  base,  5  mm.  Number 
of  inflorescences  8,  producing  80  to  120  seeds. 

Station  2 ,  quadrats  5  and  6 .  Apr.  3 ,  1 909.  The  following  samples  were 
taken  at  random,  supplying  the  attached  data  for  growth  in  1908: 

From  a  broken-off  tap-root  7  mm.  in  diameter,  2  new  shoots,  12.5 
and  13  cm.  long  by  6.5  and  7  mm.  in  diameter,  respectively. 

From  a  broken-off  tap-root  6  mm.  in  diameter,  two  new  shoots  6.5 
and  3  cm.  long  by  3.5  and  i  mm.  in  diameter,  respectively. 

From  a  lateral  root,  a  clump  of  5  stems,  each  2  cm.  long. 

Experiment   no. — Station  3    (one  quadrat).     Dec.   31,    1907.     30 

S'ants,  30  to  60  cm.  tall,  cut  off  with  a  talacho.    No  growth  observed  on 
ay  i  following,  till  which  date  there  was  no  rain.     July  16,  3  roots 
had  started.     A  number  of  roots,  including  the  3  which  had  started, 
were  taken  up  for  examination  and  the  data  tabulated  as  follows: 

TABLE  24. — Station  $  (exp.  no). 


No. 

Order. 

Position. 

Died  back. 

Diameter 
of  root 
where  cut. 

I 
a 
3 
4 
5 
6 

I 

9 

10 

Secondary  

Nearly  horizontal  .... 
Do  .            

cm. 
25 
13 
7-5 
13 

12 

3 
20 

Failed  to  start. 

mm. 

«.S 

8.0 
4.o 
5-5 
it  .0 
17.0 

S-o 
4.0 

6-5 

Do. 

Do. 

Do.. 

Do..                

Do  

Primary.  .  . 

Vertical  
Do               

....Do.. 

Secondary  l 

Horizontal 

Do 

4S°  .  • 

2.6;  merely  started 
13;  started;  shoot 
2  .  5  mm.  long. 
2io   =; 

Do 

4t» 

..Do 

Horizontal 

1  Arising  from  No.  6  at  2  cm.  from  the  top,  where  cut. 

2  Started  (in  1907  ?)  shoot  7.5  cm.  long. 

Experiment  121. — Station  4,  quadrat  i.  50  plants  in  all.  These 
were  cut  away  as  in  the  other  experiments,  Jan.  14,  1908.  May  6,  no 
growth  whatever  apparent.  A  rain-gage  was  placed  at  this  station  on 
Jan.  14.  May  6:  rainfall  registered  to  this  date,  1.52  cm. 

Apr.  5,  1909,  6  clumps  of  retonos.  This  appearance  of  new  growth 
followed  on  further  rainfall,  as  evidenced  by  the  rain-gage  of  Station  5, 
a  short  distance  away. 

Experiment  125. — Station  5,  quadrat  i.  Jan.  15,  1908.  275  plants 
cut  off  below  level  of  ground  with  a  talacho.  May  6,30  roots  have  started, 
sending  up  i  to  5  shoots  each,  but  smaller  than  those  at  Station  2 .  Between 
Jan.  15  and  May  6,  rainfall  353  mm. 

Apr.  5,  1909.  43  clumps  of  retonos.  The  increase  in  numbers  was 
the  result  of  the  additional  rainfall,  as  indicated  by  the  rain-gage,  which 
was  still  in  position,  though  standing  somewhat  obliquely.  Evidently 
some  of  the  water  had  been  lost,  as  the  oil  had  disappeared.  The  amount 
remaining,  700  c.c.,  indicated  a  total  precipitation  of  at  least  850  mm. 
Length  of  new  stems,  7  to  15  cm.,  with  diameter  of  2  to  9  mm. 

Experiment  in.  —  Station  3.     Dec.  31,  1907.     100  square  meters. 
30  plants  cut  off  at  surface  of  ground.    No  new  growths  till  after  May  i. 
July  15,  one  retofio.    Apr.  2,  1909,  two  retonos  in  all. 
5 


66  Guayule. 

The  percentages  of  removed  plants  represented  by  new  shoots  in 
all  the  above  experiments  are  as  follows: 


Per  cent. 


Experiment  115  (pulling) 6 

Experiment  1 1 1  (cutting) 6 

Experiment  114  (cutting) '23 


Per  cent. 

Experiment  no  (cutting) 10 

Experiment  121  (cutting) 12 

Experiment  125  (cutting) 15 


From  the  above  data  the  following  conclusions  may  be  drawn: 

Retonos  are  formed  much  more  easily  from  the  stock  left  after  cut- 
ting at  or  near  the  level  of  the  ground.  The  probability  that  the  plants 
removed  will  be  represented  by  new  growths  after  cutting  is  much  greater 
when  a  portion  of  the  stem  at  the  top  of  the  tap-root  is  left.  This  is  due, 
of  course,  to  the  presence  of  numerous  dormant  buds. 

The  promptness  with  which  retonos  start  after  cutting  away  the 
plants  depends,  in  the  absence  of  sufficient  soil-moisture,  upon  the  rain- 
fall. It  is  worthy  of  note  (i)  that  these  retonos  may  start  slowly  before 
the  advent  of  rain,  and  (2)  that  the  roots  may  die  back  at  least  13  cm. 
before  starting.  Root  No.  i,  experiment  no,  had  died  back  25  cm.  dur- 
ing six  and  a  half  months,  that  is,  at  the  rate  of  about  4  cm.  per  month, 
and  it  finally  failed  to  start.  It  was  a  very  dry  period,  and  this  long 
tenacity  of  life  illustrates  in  a  striking  way  the  physiological  resistance 
of  these  roots  in  desert  conditions. 

While  this  degree  of  hardiness  would  serve  very  effectively  to  pre- 
serve the  species  under  unfavorable  circumstances,  it  is  evident  from  our 
figures  that  the  number  of  new  plants  produced  is  not  as  great  as  is  gen- 
erally supposed.  The  best  result  obtained  (exp.  114)  indicates  that  under 
the  conditions  surrounding  this  experiment  scarcely  more  than  25  per 
cent  of  the  original  stand  may  be  expected.  It  is  a  matter  for  satisfac- 
tion, however,  that  even  under  the  most  drastic  treatment  a  field  of 
guayule  may  be  expected  to  reestablish  itself  in  the  course  of  time,  since 
the  new  growths  will  in  a  short  time  be  able  to  produce  seed  and  these 
will  contribute  to  the  repopulation  of  the  area. 

In  April  1909,  two  areas  were  visited  from  which  the  guayule  had 
been  removed  by  pulling  up  the  shrub.  It  appeared  that  only  the  larger 
plants  had  been  removed,  and  that  both  places  still  contained  the  natural 
growth  of  smaller  plants.  The  point  of  interest  in  this  connection  is  that 
in  one  of  the  areas,  the  Lomerio  de  Zorrillos,  it  was  very  easy  to  find 
broken-off  roots  which  had  started  to  grow  again,  and  retonos  of  various 
sizes  up  to  8  cm.  were  found.  In  the  other  area,  in  the  Sierra  de  Ramirez, 
the  ground  was  very  hard  and  the  peons  found  difficulty  in  pulling  the 
plants  out.  Instead,  they  had  twisted  them  off  just  above  the  surface, 
and  from  the  butts  remaining,  with  very  few  exceptions,  new  shoots  had 
grown  during  the  season  of  1908,  these  measuring  from  3  to  8  cm.  in 
height.  This  parallels  the  behavior  of  plants  cut  off  at  some  distance 
above  the  surface  of  the  ground. 

Experiment  60. — Station  2,  quadrat  i.  25  square  meters.  Nov.  5, 
1907.  140  plants  cut  off  at  a  height  of  8  to  10  cm.  above  the  sur- 
face of  ground. 

1  The  total  number  was  not  determined  in  April  1909,  but  would  doubtless 
have  indicated  a  larger  percentage. 


Reproduction.  67 

Jan.  6,  1908.    Many  buds  2  mm.  long. 

Feb.  18.    All  but  5  plants  budded.    Longest  leaves,  25  mm. 

Mar.  29.    Little  change.    Longest  leaves,  30  mm. 

July  28.  5  plants  dead.    Longest  stems  of  new  shoots,  7  cm. 

Sept.  12.    12  dead  altogether.    New  shoots  10  to  15  cm.  long.    Plenty 

of  flowers.    Some  plants  have  the  appearance  of  witches'  broom. 
Apr.  3,  1909.     13  dead.    Maximum  stem-growth,  20  cm.;  minimum, 

3  to  5  cm.    New  shoots  in  several  cases  killed  by  drought  (plate 

1 6,  figs.  A  to  C). 
Experiment  56. — Station  i,  quadrat  2  (25  square  meters).  All  plants  cut 

off  as  in  experiment  60,  Nov.  5,  1907. 
Jan.  3,  1908.    No  growth. 
May  29.     New  stems  (upwards  of  15  mm.  long,  4  mm.  diameter) 

on  the  majority  of  cut  stems. 

Apr.  3,  1909.    Maximum  growth,  10  cm.  stem-length. 
Experiment  126. — Station  5,  quadrat  2.    Jan.  15,  1908.    All  plants  cut 

at  15  cm.  above  ground. 
May  6.    Nearly  all  well  budded. 
Apr.  5,  1909.    New  shoots  10  to  15  mm.  long;  flowered  well  in  1908. 

From  the  rainfall  data  it  appears  conclusive  that  the  best  time  to  cut 
guayule,  with  reference  to  reproduction  by  retonos,  is  just  before  and  dur- 
ing the  rainy  season.  As  we  shall  see,  however  (Chapter  V) ,  this  is  the 
period  of  active  growth,  and  the  rate  at  which  the  accumulation  of  rub- 
ber takes  place  is  such  as  to  indicate  that  the  practice  of  removing  guay- 
ule at  this  time  is  not  advisable.  Therefore,  other  considerations  aside 
(such  as  competition  with  other  plants) ,  the  removal  of  guayule  even  dur- 
ing the  most  trying  seasons  will  not  exterminate  the  plant,  except  on  re- 
stricted areas  which  may  be  rehabilitated  by  spreading  through  seed.  It  is 
scarcely  to  be  doubted  that  even  in  the  quadrat  of  experiment  1 2 1  a  few 
retonos  made  their  appearance  after  the  last  date  of  observation,  which, 
unavoidably,  was  before  the  summer  rains.1  Furthermore,  we  are  able  to 
say  from  observation  that  the  conditions  at  this  station  were  more  rigorous 
than  at  Station  2,  where  an  earlier  start  was  made  by  the  retonos. 

The  rate  of  growth  of  induced  retonos  will  be  seen  to  exceed  the 
initial  growth  of  seedlings.  The  stem-growth  for  the  growing-season  of 

1908,  as  shown  by  observations  taken  on  the  above  experiments  in  April 

1909,  was  upwards  of  15  cm.,  the  average  amount  of  growth  falling  some- 
where near  to  8  cm.    The  retono  in  plate  9,  fig.  B,  plant  No.  10,  made  a 
stem-length  of  10  cm.  in  about  three  months,  and  would  probably  have 
made  more  growth  had  it  been  allowed  to  remain. 

As  between  the  pulling  and  cutting  methods  of  gathering  guayule, 
there  can  be  no  two  ideas  as  to  the  relative  effect  upon  the  rate  of  repro- 
duction by  means  of  retonos.  In  adjoining  quadrats  (experiments  114 
and  1 1 5),  in  which  it  so  happened  that  the  same  number  of  plants  was 
removed,  in  the  one  by  cutting  and  in  the  other  by  pulling,  the  clumps 
of  retonos  were  as  59  to  15.  This  is  explained  by  the  fact  that  the  roots 
left  in  the  ground  when  the  shrub  is  pulled  up  are  not  only  fewer  in  num- 

1  After  this  was  written  this  quadrat  was  visited  on  April  5,  1909,  and  it  is  of 
interest  to  note  that  the  belief  expressed  was  substantiated.  See  experiment  121, 
above,  on  page  65. 


68  Guayule. 

her  but  smaller  than  those  left  when  the  shrub  is  cut.  The  larger  break 
off  further  in  the  ground  and  are  therefore  less  favorably  placed  for  starting 
afresh.  The  disadvantage  of  the  cutting  method  in  the  eyes  of  those  who 
are  in  pursuit  of  the  greatest  possible  initial  return  is  that  less  tonnage 
per  acre  is  obtained,  a  loss,  however,  which  would  be  made  good  many 
times  in  new  plants  if  the  roots  were  properly  cut  and  allowed  to  remain. 

SEED. 

VIABILITY. 

The  seeds  of  guayule  appear  to  have  a  fairly  long  period  of  vitality , 
a  conclusion,  however,  which  is  inferential  and  has  not  been  demonstrated 
by  direct  experiment.  The  view  is  based  on  the  following  experiment 
(exp.  78):  On  November  23,  1907,  a  lot  of  trays  (such  as  are  shown  in 
plate  45)  were  filled  with  paper  tubes  of  i  square  inch  transverse  section. 
The  trays  were  then  filled  with  soil  made  up  of  half  and  half  garden  soil 
and  old  dry  manure  from  a  horse  corral.  In  the  top  of  each  tube  were 
sown  20  to  30  seeds.  The  trays  were  watered  abundantly  by  subirriga- 
tion,  it  being  the  purpose  to  try  the  method  of  using  the  trays  with  paper 
tubes  for  wholesale  germination.  So  far  as  this  was  concerned,  the  experi- 
ment was  a  failure,  but  it  served  to  contribute  to  our  knowledge  of  seed 
vitality.  The  very  dry  season  made  it  very  difficult  to  keep  the  surface 
soil  moist,  and  as  a  result  of  alternate  drying  and  wetting  the  upper  part 
of  the  soil  became  caked  and  there  was  considerable  efflorescence  of  salts. 
The  soil  below  became  soggy  and  sour,  and  fungi  permeated  the  soil  and 
the  paper  of  the  tubes.  Very  few  seeds  germinated,  not  more  than  one 
or  two  in  each  tray,  partly,  as  was  later  determined,  because  of  the  char- 
acter of  the  soil,  and  partly  because  of  the  prevailing  low  temperatures. 
The  trays  lay  thus,  occasionally  wet  by  showers,  till  the  following  July, 
when  a  large  number  of  seeds  started  to  germinate.  In  one  tray  138  tubes 
had  seedlings,  from  one  to  eight  in  each.  By  July  25  the  seedlings  had 
developed  two  foliage  leaves,  and  by  August  28  a  stem-growth  of  5  cm. 
was  not  exceptional,  with  leaves  5  cm.  long.  Some  plants  had  at  this 
date  as  many  as  seven  foliage  leaves.  Thus  it  will  be  seen  that  the  seeds 
which  germinated  did  so  after  six  months'  exposure  to  conditions  about 
as  bad  as  could  be  imagined,  being  alternately  wet  and  dry,  in  a  sour 
soil,  and  open  to  the  attacks  of  fungi.  The  germination  in  trays  favorably 
placed  with  respect  to  shade  was  upwards  of  13  per  cent  of  viable  seed, 
as  nearly  as  may  be  calculated.1  It  may  therefore  be  concluded  that  the 
seed  of  guayule,  being  neither  very  short-lived  nor  very  sensitive  to  unto- 
ward conditions,  is,  from  a  biological  point  of  view,  quite  efficient  for  the 
preservation  of  the  species.2 

It  is,  in  the  nature  of  the  case,  well-nigh  impossible  to  determine 
the  percentage  of  germination  of  nature-strewn  seed,  but  one  successful 
experiment  affords  us  exact  data  (experiment  192).  On  May  30,  after  a 

1  Critical  germination  tests  to  determine  the  viability  of  seed  have  been 
made  by  Kirkwood  (19100),  who  finds  the  germinations  to  scarcely  exceed  14  per  cent, 
and  that  after  eight  months  there  is  a  marked  drop  in  viability. 

2  Ready  germination  front*  seed  collected  during  the  summer  of  1908  was 
obtained  in  July  1909,  at  Auburn,  Ala. 


A--C.  New  growth*  after  pollarding.     A,  February  18,  1908; 

B,  March  29,  1908;  Q  April  5,  1909. 
D.  Seedlings  in  limestone  soil;  E,  in  "garden"  soil. 


;/* 


A.  Minimum,  average,  and  maximum  seedlings.      (Station  2,  quadrat  4.) 

B.  Irrigated  plant,  two  years  old,  from  a  stock.      April  1909.     Cedros. 


Reproduction.  69 

rain  of  16.8  mm.  during  the  preceding  night,  4  ounces  of  seed  (including 
chaff)  were  sown  at  Station  7,  in  5  rows,  each  a  meter  long.  The  ground 
was  previously  cleared  of  all  plants  and  thus  loosened,  and,  the  seed 
having  been  left  uncovered,  the  seedlings  were  exposed  to  full  insolation. 
On  September  9  following,  119  seedlings  were  counted.1  These  compared 
favorably  in  appearance  and  size  with  other  seedlings  found  growing 
spontaneously  in  the  surrounding  area.  The  seed  was  sown  more  thickly 
than  would  occur  in  nature,  and  the  number  of  seedlings  was  also  much 
greater,  and  far  too  great  for  their  normal  development.2 

Comparison  of  these  results  with  those  obtained  by  observation  of 
germination  in  irrigated  ground  affords  considerable  interest.  About 
150  plants,  placed  in  a  small  patch  of  ground  by  Mr.  C.  T.  Andrews  in 
the  spring  of  1907,  flowered  freely  during  that  and  the  following  year. 
A  very  large  number  of  seeds  must  have  been  disseminated,  notwith- 
standing a  good  deal  had  been  gathered,  of  which  fully  30  per  cent  were 
viable.  During  the  summer  of  1908,  at  the  time  (June)  when  seed  was 
germinating  in  the  surrounding  region  under  natural  conditions,  some 
seedlings  were  observed.  About  50  were  counted,  but  in  the  whole 
area  (o.i  acre)  there  could  hardly  have  been  more  than  a  few  hundred 
at  the  outside.  Nor  did  they  grow  as  well  as  field  seedlings,  perhaps 
because  of  the  rapid  drying  of  the  superficial  layers  of  soil.  The  percent- 
age of  germination  here  must  therefore  have  been  exceedingly  small, 
and  much  less  than  that  which  occurred  in  experiment  192  above  de- 
scribed, and  also  than  that  which  takes  place  in  nature,  if  we  may  judge 
by  the  numbers  of  seedlings  actually  found  in  the  field  in  the  summer 
of  1908.  The  following  observations  are  pertinent  here: 

(1)  Station  3.     June  1908.     In  areas  of  i  square  meter,  representative 

counts  of  8  and  14.  April  1909,  23  living  seedlings  of  1908  were 
found  on  the  whole  quadrat  (100  square  meters). 

(2)  The  region  about  Stations  7  and  8.    On  June  24  a  large  number  of 

seedlings  was  seen. 

(3)  Station  2,  quadrats  5  and  6.    Sept.  12.    Four  seedlings  10  cm.  apart. 

Nearby  6  seedlings  10  cm.  apart.  Several  counts  showed  about 
20  plantlets  per  square  meter.  None  on  previous  visit  to  this 
station,  July  28. 

(4)  In  i  square  foot  on  the  same  area,  6  well-grown  seedlings.    Sept. 

12,  1908. 

(5)  In  a  wire-fenced  quadrat  which  was  cleared  of  all  plants   (other 

than  guayule)  by  Mr.  C.  T.  Andrews  early  in  1907,  5  miles  north 
of  Cedros  in  an  open  plain,  leaving  one  tall  guayule  plant  in  the 
middle,  no  seedlings  appeared  till  after  June.  In  September  29 
seedlings  were  found  within  6  feet  of  the  plant,  chiefly  in  one 
direction.  One  mariola  seedling  was  found. 

(6)  Station  8,  quadrat  i  (100  square  meters).    24  seedlings,   Sept.  1908. 

(7)  In  4  square  feet,  on  a  loma  north  of  Cedros,  near  Station  8,  Aug.  8, 

31  seedlings,  all  of  1908  except  one  of  1907.  This  number  in- 
cluded one  of  Parthenium  hysterophorus. 

1  The  largest  of  these  had  a  stem  (epicotyl)  i  cm.  long,  with  leaves  4.7  cm. 
long  by  1.5  cm.  broad. 

3  In  April  1909  it  was  found  that  all  the  seedlings  had  been  destroyed  by  goats. 


70 


Guayule. 


(8)  Near  this  place  22  seedlings  were  collected  from  two  areas,  each  of 

12  square  inches;  i  of  1906,  8  of  1907,  and  13  of  1908. 

(9)  Station  2,  quadrat  4.    April  1909.     281  living  seedlings,  all  of  which 

germinated  during  the  growing-season  of  1908,  were  collected  on 
100  square  meters  (plate  17,  fig.  A). 

(10)  Endlich  reports  finding  "as  many  as  50  young  plants  around  full- 

grown  trees"  (335,  1905,  Eng.  tr.).  Such  a  large  number  is  not 
common,  but  it  is  not  unusual  to  find  25  seedlings  with  two  foliage 
leaves  about  the  base  of  a  single  plant. 

From  such  observations  it  is  clear  that  in  particular  areas  one  may 
find  by  chance  many  more  seedlings  than  could  by  any  fortune  develop 
into  mature  plants.  Other  areas,  however,  are  quite  bare  of  them.  Again, 
many  seedlings  which  get  started  die  in  the  course  of  time,  and  there  can 
be  no  doubt  that  the  percentage  of  deaths  is  great.  Counting  seedlings, 
therefore,  is  not  a  dependable  method  of  determining  the  rate  of  repopu- 
lation.  For  this  purpose  it  is  necessary  to  make  a  census  of  sample  quad- 
rats, making  as  careful  estimates  as  possible  of  the  sizes  and  ages  of  the 
plants.  The  data  in  tables  4  to  13  afford  such  a  census.  They  are  summa- 
rized in  table  25  and  are  further  displayed  graphically  in  fig.  12. 


TABLE  25.- 


llassification  of  guayule  plants  from  seed  according  to  weight  on  various 
quadrats  indicated. 


Quadrat. 

4  Ibs.  or 
more. 

3  Ibs.  or    !    2  Ibs.  or 
more.     !      more. 

i  lb.  or 
more. 

i  lb.  or 
more. 

Less  than 
ilb. 

Table  4... 

0 

0                            0 

IO 

IOO 

585 

6  

0 

7                    13 

IS 

II 

26 

7  

7 

5                    13 

40 

4 

10 

8  

0 

2                         12 

20 

9 

86 

:9  

0 

0                            0(?) 

o(?) 

25°(?) 

755 

10  .    .  . 

2 

•2                                     C 

6 

ii  

12  ...                       ... 

0 

o 

0                          I 
O                            I 

28 

23 

59 

90 
232 

13  

0 

I                          12 

45 

53 

166 

1  It  is  to  be  recalled  that  the  larger  plants  had  previously  been  removed  from  this  quadrat.  The 
estimate  marked  doubtful  is  based  on  the  figures  of  adjoining  quadrats,  and  can  only  be  approximate. 

It  is  clear  that  the  ratios  between  small  and  large  plants,  as  shown 
in  table  25,  indicate  very  different  degrees  of  efficacy  in  reproduction 
commencing  from  the  seed.  This  method  is  the  best  available  in  the 
absence  of  actual  counts  of  seedlings  year  by  year,  obviously  not  practi- 
cable. A  few  such  counts,  for  future  comparison,  are  given  in  table  26. 

TABLE  26. 


Seedlings 
of  1908. 

When  counted. 

Station 

8   quadrat  i 

1008 

Station 

3    quadrat  i 

2T. 

Apr.  2    1909 

Station 

2,  quadrat  4 

281 

Apr.  T.    i  QOO 

These  few  data,  the  difficulty  of  obtaining  which,  ofl  account  of  vari- 
ous circumstances,  was  very  great,  have  only  suggestive  significance.  It  is 


Reproduction. 


71 


obvious  that  in  the  last  station  reproduction  by  seedlings  is  relatively  very 
good,  especially  as  the  counts  were  made  at  the  close  of  a  long  drought. 
A  condition  such  as  this  might,  in  the  light  of  table  25,  be  expected  to 
lead  to  a  good  stand  of  guayule.  From  a  consideration  of  the  curves 
based  upon  table  2  5 ,  some  further  points  of  interest  are  discovered.  There 
is  a  large  falling  off  in  numbers  of  plants  between  the  average  weight 
of  about  4  and  1 2  ounces.  This,  as  seen  in  the  curves  on  pages  87  and  88 , 
is  the  period,  approximately,  of  maximum  rate  of  growth,  viz,  between 
8  to  10  and  13  to  15  years  of  age,  during  which  time  there  is  a  loss  of 
total  weight  of  about  one-fourth  to  one-third,  as  nearly  as  we  may  calcu- 
late. From  the  nature  of  the  conditions,  many  of  which  are  undetermi- 
nable, such  calculations  can  be  only  loosely  approximate,  but  it  can  hardly 
be  doubted  that,  if  the  rate  of  reproduction  by  seed  from  plants,  say 
from  6  to  8  ounces  in  weight,  can  be  depended  upon  quantitatively  as 


0-%.  yz-\  "-2  2-3  3-4-  4- 

POUNOS 

FIG.  12. — The  relative  numbers  of  various-sized  plants  on  different  quadrats.     The  numbers 
at  the  ends  of  curves  refer  to  the  tables  corresponding. 

indicated  in  the  table  under  consideration,  it  is  an  economic  loss  to  allow 
plants  larger  than  these  to  remain.  From  this  point  of  view  alone  it 
may  not  pay  to  allow  the  plants  to  remain  after  the  age  indicated  by 
the  weight  of  6  to  8  ounces  has  been  attained,  as  the  numbers  which  die  off 
are  great  enough  to  cause  a  considerable  falling  off  of  total  weight. 

The  data  show  also  that  the  initial  monetary  return  from  a  harvest- 
ing of  guayule  may  be  as  great  or  greater  from  a  stand  of  a  few  large 
individuals,  but  the  areas  with  large  numbers  of  smaller  plants  give 
promise  of  future  returns. 

An  important  desideratum  is  to  determine  how  to  improve  these 
conditions.  Here,  let  us  say,  is  a  good  field  of  guayule,  as  regards  first 
returns.  The  bulk  of  the  weight  is  in  large  plants,  and  the  small  ones 
are  too  few  for  a  ready  reseeding  of  the  area  after  depletion.  It  is  hardly 
too  much  to  say  that  vast  areas  are  in  this  condition.  What  may  be 


72  Guayule. 

done  to  increase  their  productivity  is  still  a  question  for  experimental 
determination,  but  seeding  in  favorable  years  by  means  of  seed  from 
densely  grown  areas  would  be  distinctly  beneficial.  The  importance  of 
seed  is  so  great  that  in  the  harvesting  of  shrub  the  practice  of  leaving 
large  plants  for  the  purpose  of  producing  seed  should  in  all  circumstances 
be  initiated.  As  a  practical  question  of  economics,  the  difficulties  of 
time  and  distance  in  the  desert  are  so  great,  not  to  mention  those  arising 
in  connection  with  climatic  irregularities,  that  any  attempts  to  better 
conditions  over  wide  areas  are  fraught  with  expense  which  may  not  be 
considered  as  warranted  by  those  interested. 

COMPARATIVE   ABILITY   TO    GERMINATE   IN   THE    FIELD. 

The  ability  to  germinate  promptly,  to  attain  a  condition  of  physio- 
logical resistance,  is  of  prime  importance  to  desert  plants,  and  very  much 
more  important  to  them  than  to  plants  which  are  more  favorably  placed 
with  reference  to  water-supply  (Ganong,  1907;  Lloyd,  19090).  So  far  as 
the  question  of  germination  is  concerned  the  evidence  is  not  forthcoming 
that  desert  plants  exhibit  more  indifference  to  initial  water-supply  than 
others  (Livingston,  1906).  For  the  rest,  as  for  further  elucidation  of 
this  problem,  much  comparative  study  is  necessary.  There  seems  to  be 
little  doubt,  however,  that  the  rate  at  which  physiological  resistance  is 
acquired  and  the  amount  of  this  resistance  are  very  different  in  different 
plants.  For  example,  the  seedlings  of  many  succulents  soon  acquire 
the  characters  of  the  parents,  the  cacti  (Ganong,  1898)  being  notable 
examples  of  this.  This  must  be  of  no  small  weight  as  a  factor  in  enabling 
young  plants  to  withstand  the  rigors  of  drought,  though  this  very  cir- 
cumstance in  the  cacti  opens  them  to  the  attacks  of  animals  (MacDougal, 
1910),  so  that  millions  of  seedlings  are  eaten,  affording  both  food  and 
water  to  desert  animals. 

As  has  been  shown,  and  as  will  be  further  developed  in  the  following 
chapter,  the  guayule  seedling  offers  no  exception  to  the  rule  that  desert 
plants  need  an  abundance  of  water  during  the  period  of  germination. 
Observation  in  the  field  indicates  further  that  marked  readiness  in  ger- 
mination is  not  in  any  way  indicative  of  adaptation  to  desert  conditions. 
A  field  test  of  the  germinating  ability  of  guayule  in  comparison  with 
that  of  alfilaria  (Station  7,  May  30,  1908,  exp.  139)  showed  that  about 
3  per  cent  of  the  seed  of  the  latter  germinated,  while  about  0.2  or  0.3  per 
cent  of  guayule  succeeded  in  getting  a  foothold  in  the  same  place  under 
the  same  conditions.  These  figures  are  probably  too  low  for  both  plants, 
inasmuch  as  ants  were  observed  carrying  off  seed  on  each  occasion  that 
the  station  was  visited:  This  test,  however,  may  indicate  the  direction 
in  which  research  may  contribute  toward  the  explanation  of  the  success 
which  the  alfilaria  has  had  in  invading  desert  territory. 

A  further  observation  was  made  at  Camacho,  on  the  Mexican  Central 
Railway,  on  the  Hacienda  de  Cedros,  a  point  for  the  shipment  of  guayule, 
where  a  stack-ground  had  been  kept  supplied  with  shrub  from  the  neigh- 
boring region.  It  is  customary  to  bring  in  the  shrub  in  loose  bundles  on 
the  backs  of  burros  or  in  carts  of  various  sizes  and  kinds  to  these  ship- 
ping-points, there  to  be  made  up  into  bales  for  handling  on  the  railroad. 


Seedlings  growing  in  different  soils:  A,  May  25.  1908;  B,  April  13.  1908. 


PLATE   19 


A.  (1)  Root-cutting;  (2  to  4)  Sectional  root-stem  cuttings  (Exp.  146). 

B.  Seedlings  grown  in  different  soils,  August  1908. 


Reproduction.  73 

Countless  numbers  of  seeds  are  therefore  strewn  upon  the  ground,  and 
indeed  the  new  plants  of  guayule  which  spring  up  on  these  stack-grounds 
sometimes  afford  valuable  data  on  the  rate  of  growth,  although  decep- 
tive notions  as  to  the  numbers  of  plants  which  may  be  expected  are  some- 
times acquired.  The  conditions  of  a  stack-ground  are,  at  Camacho  at 
least,  a  rather  severe  test,  as  it  lies  out  in  the  open,  dry  plain,  exposed  to 
full  sunlight.  At  the  same  time,  the  surface  of  the  soil  is  mulched  by  the 
debris  of  broken-off  guayule  twigs,  and  thus  the  conditions  are  amelior- 
ated. Shipments,  which  had  been  made  for  a  year  or  longer,  ceased  in 
the  fall  of  1907,  and  the  spot  was  under  occasional  observation  for  some 
time  before  and  from  that  time  on,  till  the  following  September.  Al- 
though it  is  known  that  much  guayule  was  brought  in  in  flowering  con- 
dition and  that  seed  must  have  been  dropped  in  large  quantities,  the 
conditions  for  germination,  especially  the  meager  rainfall,  were  not  favor- 
able for  guayule,  though  the  seeds  of  the  plants  in  the  following  list 
were  found  in  all  conditions  of  development  in  June  and  July  of  1908: l 

Helianthus  sp.     15  to  18  inches  tall  and  many  in  flower. 

Amaranthus,  2  species.     Plants  3  inches  tall. 

Cassia  ("coco").     Many  mature  plants  in  flower. 

Prosopis  seedlings  with  the  plumule  well  developed. 

Euphorbia  of  2  species.     Mats  10  inches  in  diameter. 

Solatium  sp. 

A  cucurbitaceous  vine. 

Chenopodium,  a  species  with  broad  deltoid  leaves. 

Grasses  of  4  species. 

Spheralcea,  mature  plants. 

In  addition  to  these  seedlings,  the  roots  of  Prosopis  and  Covillea, 
which  had  been  cut  off  in  preparing  the  ground  for  stacking,  had  sent  up 
shoots  from  10  to  20  inches  in  length.  That  not  a  single  guayule  plant 
sprang  up  is  at  first  surprising,  not  to  say  disconcerting,  but  in  the  light 
of  experimental  evidence  it  becomes  clear  that  the  guayule  germinates 
only  under  highly  favorable  conditions.  For  some  time  it  has  a  low  de- 
gree of  resistance,  and  is  in  point  of  fact  of  distinctly  mesophytic  charac- 
ter. It  is  only  when  due  regard  to  this  is  had  that  the  maximum  rate  of 
germination  may  be  expected  under  cultural  conditions. 

HABITATS    OP    SEEDLINGS. 

The  particular  preference  of  the  guayule  for  certain  germination 
habitats  is  of  importance  in  its  bearing  on  the  effect  of  clearing  land  of 
other  plants.  It  has  been  repeatedly  observed  by  the  investigators  at 
the  Desert  Botanical  Laboratory,  and  by  myself  in  Zacatecas,  that  there 
are  usually  to  be  found  many  more  plants  of  smaller  size  growing  in  the 
partial  shade  of  shrubs  than  elsewhere,  and  it  is  to  the  protective  effect 
of  this  shade  that  the  many  curious  juxtapositions  of  perennial  plants 
may  be  referred.  An  example  of  this  is  the  frequently  seen  saguaro 
(Carnegeia  gigantea) ,  standing  in  a  position  indicating  that  it  germinated 
in  the  shade  of  a  palo  verde  (Parkinsonia  microphylld)  or  some  other 
shrubby  species. 

As  regards  the  guayule,  Endlich  (1905)  speaks  of  "the  large  numbers 
of  young  plants  sometimes  found  surrounding  the  older  trees  *  *  *  in  the 

1  Every  annual  had  disappeared  by  April  1909. 


74  Guayule. 

territory  around  Jimulco,  for  instance,  as  many  as  50  young  plants  have 
been  found  around  full-grown  trees."  But,  on  the  other  hand,  speaking 
of  the  occurrence  of  young  plants  in  supposedly  very  unfavorable  spots, 
Endlich  explains  this  by  saying  that  "it  is  likely  that  they  have  been 
developed  from  such  seeds  as  were  either  stamped  into  the  ground  by 
goats  (as  these  animals  are  the  ones  which  commonly  graze  in  the  guay- 
ule  territories),  or  had  been  dropped  by  these  animals  and  thus  found 
favorable  conditions  of  development  in  the  animal  excrements.  It 
would,  in  fact,  be  difficult  to  find  any  other  explanation  for  the  enormous 
growth  of  the  guayule  plant  in  small,  isolated  places  (having  usually 
the  size  of  the  resting-places  of  the  herds  of  goats)  *  *  *  ." 

As  to  the  supposedly  favorable  conditions  afforded  by  animal  excre- 
ment, it  may  well  be  doubted  that  these  are  more  so  than  the  soil  itself 
affords.  Experiments  have  shown  that  soil  at  all  rich  in  humus  derived 
from  manure  is  distinctly  unfavorable  for  healthy  germination.  Even 
"garden"  soil  at  Cedros,  with  no  addition  of  manure,  is  less  favorable 
than  the  unaltered  lime-charged  soil  of  the  normal  guayule  habitat  (plate 
1 6,  figures  D  and  E).  Even  after  thorough  leaching  from  exposure  the 
possible  advantages  are  hardly  important,  and,  at  all  events,  in  such 
situations  the  seeds  and  seedlings  have  no  advantage  of  shade,  as  the 
herding-spots  of  goats  are  usually  bare  of  vegetation.  Nor  can  the 
stamping  into  the  soil  by  these  animals  have  any  value,  as  the  seeds  ger- 
minate well  only  with  very  shallow  soil  covering,  as  much  as  2  mm. 
depth  being  enough  to  show  a  marked  decrease  in  germination.1  It  would 
seem,  therefore,  that  if  Endlich's  observations  are  correct  as  to  the  occur- 
rence of  guayule  seedlings  in  such  situations,  it  is  safe  to  infer  that  the 
rainfall  conditions  are,  on  occasion,  such  as  to  make  ready  germination 
and  early  growth  possible  for  a  good  percentage  of  seeds  even  in  open 
bare  spots  where  no  advantage  of  shade  is  offered.  My  own  observations, 
at  any  rate,  sustain  this  view.  Experiment  139  (see  p.  72)  is  a  case  in 
point,  and  the  results  were  supported  by  general  observation  during  the 
summer  of  1908,  when  there  was  a  fairly  generous  if  not  a  maximum  field- 
germination.  The  net  result  of  this  season  is  indicated  by  the  numbers 
of  seedlings  observed  in  April  1909  (see  p.  70).  These  are  known  to 
have  germinated  at  or  during  the  growing-season  of  1908,  and  had  suc- 
cessfully sustained  prolonged  drought  till  the  time  of  observation.  At 
no  other  point  was  there  seen  a  better  crop  of  seedlings  at  the  age  of 
these,  and  they  germinated  without  the  least  protection,  as  the  quadrat 
had  been  completely  denuded.2 

Nevertheless,  when  seedlings  are  observed  in  the  field  at  other  than 
favorable  seasons,  it  is  frequently  noticed  that  the  larger  numbers  are 
in  the  protective  shade  of  other  plants;  but  this  is  not  peculiar  to  the 
guayule  alone.  The  explanation,  we  believe,  is  not  that  the  guayule 
seedling  is  ombrophile,  but  that  the  eliminating  effect  of  the  drought 
period  subsequent  to  a  period  of  germination  is  more  drastic  elsewhere 

1  Kirkwood,  1910. 

2  By  contrast,  it  should  be  said  that  at  Station  i  only  very  few  seedlings 
were  found  on  a  large  area  denuded  of  all  plants  save  small  guayule.     As  goats 
had  been  pastured  here,  however,  it  is  impossible  to  draw  any  conclusions. 


Reproduction.  75 

than  in  the  shade.  Thus,  in  February  1908,  small  seedlings  with  i  to  5 
foliage  leaves  could  be  found  beneath  the  shade  of  an  occasional  larger 
guayule  plant,  but  in  a  precarious  condition,  some  dead,  others  moribund, 
and  plainly  the  survivors  of  the  crop  of  seedlings  of  late  in  1907,  the  chief 
part  of  which  had  succumbed  to  the  very  severe  conditions  already  noted 
as  having  prevailed  at  Cedros  at  that  time.  As  bearing  upon  this  ques- 
tion, we  may  note  the  meager  occurrence  of  Opuntia  leptocaulis  in  south- 
ern Arizona,  where  it  is  scarcely  to  be  found  except  protected  by  some 
plant,  while  it  grows  in  the  open  in  great  abundance  in  Zacatecas.  It 
appears  evident  that  in  Arizona  the  conditions  for  its  persistence,  except 
when  it  is  more  or  less  protected  by  other  plants,  are  too  severe.  No  such 
relation  has  been  observed  in  Zacatecas,  and  it  would  seem  that  the  cli- 
matic conditions  there  are  distinctly  more  favorable  for  this  plant. 

It  would  therefore  appear  safe,  if  desirable,  to  clear  guayule  fields 
of  the  major  part  of  other  vegetation.  An  occasional  year  may  be  ex- 
pected when  the  rate  of  germination  will  go  far  toward  producing  a  good 
stand  of  young  plants.  Those  already  growing  will  offer  protection  to 
the  younger  brood,  and  the  larger  area  available  for  guayule  plants  will 
in  part  compensate  for  the  loss  of  shade  given  by  other  vegetation.  It 
would  not  be  advisable,  however,  to  remove  the  occasional  palma  saman- 
doca  (Samuella  carnerosd) ,  which  produces  fiber,  or  the  large  barrel  cacti 
("  bisnaga  burra  "  and  "  bisnaga  colorada  ") ,  as  they  are  heavy  plants  and 
neither  spread  with  appreciable  rapidity  nor  occupy  more  than  a  negli- 
gible fraction  of  the  ground  (plate  i,  fig.  A).  This  principle  of  practice 
is,  however,  in  the  nature  of  a  compromise,  and  rests  upon  an  estimated 
balance  of  circumstances.  A  more  correct  estimate  of  probabilities  could 
be  based  only  upon  longer  observation  under  experimental  conditions. 

RATE  OF  REPRODUCTION  AND  OF  GROWTH. 
RATE  OF  GROWTH  DURING  GERMINATION. 

This  period  may  be  divided  into  a  period  of  tissue  expansion  and 
one  of  induration.  At  the  close  of  expansion,  which  begins  in  about  a 
week's  time  after  sufficient  rain,  and  occupies  a  second  week,  the  seedling 
is  tender,  the  hypocotyl  white  and  translucent,  and  the  cotyledons  green 
(fig.  8).  The  cuticle  then  thickens,  and  red  color  is  developed  in  the 
epidermis  of  the  hypocotyl  and  under  surface  of  the  cotyledons,  while  the 
latter  become  darker  green  and  more  indurated.  This  occupies  a  third 
week,  when,  if  no  untoward  circumstance  interferes,  the  first  foliage 
leaves  develop.  Even  under  the  best  of  conditions  this  period  of  three 
weeks  will  scarcely  be  shortened. 

The  further  seedling  development  is  a  direct  function,  other  things 
being  equal,  of  the  rainfall,  the  maximum  potentiality,  it  may  safely  be 
said,  never  being  exerted  by  field  plants.  This  apparently  extremest 
limit  of  growth  for  a  seedling  was  reached  by  one  of  two  particular  indi- 
viduals under  cultivation,  and  constantly  supplied  with  an  abundance 
of  water.  The  height  of  this  plant  when  the  rhythm-limit  was  reached, 
as  indicated  by  cessation  of  growth,  was  25  cm.,  and  it  had  a  spread  of 
22  cm.  It  was  a  fully-developed  specimen,  in  which  each  branch  reached 


76 


Guayuh 


its  proportionate  size.  It  flowered  freely  and  produced  fully  2000  seeds 
(exp.  1390;  plates  18  to  20).  The  time  occupied  in  its  growth  was  about 
four  months. 

We  may  now  offer  data  (table  27)  derived  by  field  observation  during 
the  growing  season  of  1908,  which  was  a  favorable  year,  though  not  per- 
haps exceptionally  so.  The  rate  of  growth  during  germination  is  indi- 
cated by  the  measurements  of  seedlings  from  Station  3,  collected  July 
15,  1908.  They  were  two  to  three  weeks  old. 

TABLE  27. — Rate  of  growth  during  period  of  germination. 


Hypocotyl. 

Cotyledon. 

Length. 

Diameter. 

Length. 

Breadth. 

mm. 

mm. 

f>'-5 

10.  0 
II  .0 

0-75 

I  .0 
I  .0 

3-5 
2-5 
4.0 

4-5 

3 
2-5 
3-5 
4-0 

Table  28  contains  data  based  upon  the  individual  examination  of  1 1 2 
seedlings  collected  in  the  field  on  the  dates  mentioned.  The  measure- 
ments are  exclusive  of  the  hypocotyl,  which  measures  about  10  mm.  on 
the  average. 

TABLE  28. — Amount  of  growth  of  seedlings  in  the  first  season  of  growth. 


No.  of 

Height  includ- 
ing leaves. 

Length  of  stem,   i 

Collected. 

seed-                                      (                                   ;      Locality. 

Notes. 

lings. 

t 

Max. 

Min. 

Ave.    Max.   Min. 

Ave. 

I 

mm. 

mm. 

mm 

mm. 

mm. 

mm. 

Feb.,        1908 

II            I        I? 

7 

10 

2 

0 

i± 

Loma    north 

Germinated  Nov.  (?), 

6-5 

of  Cedros. 

1907.   2  to  5   foliage 

leaves.      Cotyledons 

long  gone.    Plants  of 

very  slow  growth. 

June  2,    1908 

I 

70 

30 

|  Station  2  

An     exceptional    and 

very    large    seedling 

for  this  date.     Inflo- 

rescence 2.5  cm.  long. 

Aug.  8,     1908 

28 

33 

6 

20 

2 

o 

i± 

Loma    north 
of  Cedros. 

Cotyledons  still  attach- 
ed,     i   to   7   foliage 

leaves. 

Sept.  8,   1908 

24           65 

10 

35 

12 

i 

5-7 

Sta.  8,  quad- 

Good    healthy    speci- 

rat i  

mens. 

Sept.  12,  1908 

6           75 

47 

62 

17 

5 

9-6 

Station  2  

Do. 

Sept.  12,  1908 

ii          90 

50 

73 

50 

7 

24 

Station  2  2  in  flower.    Max.  stem 

diam.  3  mm. 

Sept.  12,  1908 

3i           75 

30 

45 

20 

3 

9 

Bare  quadrat 
in  plain  be- 

Good healthy  plants. 

tween   Ce- 

Grand  aver. 

14.8  ';       dros     and 

Sta.  2. 

The  seedlings  in  table  28  were  not  selected,  but  were,  in  each 
case,  all  the  seedlings  found  in  a  given  area.  Taking  those  collected  in 
September — which,  judging  by  the  behavior  of  guayule  plants  in  gen- 
eral, was  near  the  close  of  the  growing-season — we  have  an  average  rate 


Reproduction. 


77 


of  growth  of  about  14.4  mm.  in  stem-length  (epicotyl) ,  aside  from  the  small 
secondary  branches.  With  few  exceptions,  the  seedlings  of  a  month  pre- 
vious (August  8)  were  very  small,  as  indicated  in  the  table,  but  neverthe- 
less the  size  attained  by  them,  judging  from  experience  in  their  culture, 
must  have  been  the  result  of  at  least  six  weeks'  growth. 

This  was  not  the  close  of  the  growing-season,  but  I  was  fortunately 
able  to  complement  the  above  data  by  measurements  of  seedlings,  already 
mentioned  in  other  connections,  which  had  passed  completely  through 
the  growing-season  of  1908  and  been  collected1  in  April  1909,  in  a  state 
of  dormancy.  The  measurements  of  311  seedlings  were  made  by  caliper. 

Tables  29  and  30  give  the  data  for  two  quadrats;  a  third,  having  281 
seedlings,  4  of  which  are  seen  in  plate  17,  fig.  A,  is  not  given  in  detail. 

Combining  the  averages  obtained  from  tables  29  and  30  with  the  data 
for  Station  3,  quadrat  4,  obtained  at  the  same  time  as  those  of  Station  2, 
quadrat  7,  we  obtain  table  31. 


TABLE  29. — Growth  of  seedlings  which  germinated  about  Ju 
April  2,  1909.    Station  3.     All  within  100  sq 


ne  i,  1908,  and  examined 
square  meters. 


No. 

Length  of  main 
stem  above 
hypocotyl.  (Hy- 
pocotyl  10  mm.) 

Diameter  at 
base. 

Remarks. 

i 

mm. 
35 

mm. 

3-5 

Flowered,  flower  bitten  off;  branched. 

2 

r9 

3  -2 

Branches  i  to  3  mm. 

3 

9 

.0 

Unbranched. 

4 

IO-5 

.  2 

Lateral  buds  just  started. 

5 

ii 

.0 

Unbranched. 

6 

II 

.0 

Do. 

7 

5 

.8 

Two  buds  at  base  of  hypocotyl;  otherwise 

unbranched. 

8 

8 

.0 

Slender  branch  5  mm.  long  at  base  of  hypo- 

cotyl; otherwise  unbranched. 

9 

6 

•  5 

Unbranched. 

10 

4 

.0 

Do. 

1  1 

5  -2 

.0 

Do. 

12 

5-2 

.0 

Do. 

13 

5-° 

.0 

Do. 

14 

4-2 

.0 

Slightly  damaged  ;  unbranched. 

15 

4 

.8 

Unbranched. 

,Il 

3| 

3-0 

•7 

Do. 

19 

4.0 

.  2 

Do. 

20 

3  -° 

.  2 

Do. 

21 

2-5 

.0 

Do. 

22 

17.0 

.8 

Unbranched;  etiolated. 

23 

8.0 

.0 

Unbranched;  slightly  etiolated. 

Ave. 

8.1 

1.8 

23  seedlings;  average  length  of  main  stem,  excluding  Nos.  22  and  23,  which  are 
not  normal,  7.65  mm. ;  average  diameter  of  main  stem  at  base,  2  mm. 

N.B. — The  exact  age  of  the  above  seedlings  does  not  exceed  10  months.  Of  this 
period,  6J  months  were  without  rain,  beginning  with  the  middle  of  September.  All 
the  seedlings  were  alive  at  the  time  of  collection. 


In  company  with  Mr.  G.  E.  Pell,  of  New  York. 


78 


Guayule. 


TABLE  30. — Growth  of  seedlings  (all  unbranched)  which  germinated  about  June  i, 
1908;  collected  April  3,  1909.     Station  2,  quadrat  7,  100  square  meters. 


No. 

Length  of  main 
stem  exclusive 
of  hypocotyl 
(about  10  mm.). 

Diameter  at 
base. 

mm. 

mm. 

I 

4-5 

2  .0 

2 

5-° 

2  .0 

3 

5  •  ° 

2  .0 

4 

6  .  o 

2  .0 

5 

3  -5 

1-5 

6 

6  .  o 

t.3 

7 

5-o 

I  .0 

Ave. 

5-0 

i-7 

TABLE 


Length  of  stem.                                   Diameter  of  stem. 

No.  of  seedlings 
in  quadrat. 

Max. 

Min. 

Ave. 

Max. 

Min. 

Ave. 

mm. 

mm. 

mm. 

mm. 

mm. 

23 

35 

2-5 

8.1 

3-5 

0.8 

1.8 

7 

6 

3-5 

5   ° 

2  .0 

I  .0 

1-7 

281 

55 

12.6 

6.0 

0.8 

2.8 

311 

Ave. 

32 

2-5 

8-5 

3-8 

0.9 

2.18 

It  will  be  seen  that  the  average  maximum  amount  of  growth  for  the 
whole  of  the  growing-season  of  1908,  as  indicated  by  the  data  obtained  in 
April  1909,  is  8.5  mm.,  stem-length.  This,  however  (as  shown  by  table 
28),  is  less  than  the  amount  determined  by  the  measurement  of  seedlings, 
germinated  in  1908  but  collected  on  September  8  to  12  of  that  year, 
namely,  14.8  mm.  The  difference  in  favor  of  the  earlier  collections  may 
perhaps  be  explained  by  the  fact  that  care  was  not  taken  to  take  every 
seedling  in  a  given  area.  To  do  this  requires  a  minute  search,  which  was 
given  only  in  April  1909.  It  is  not  improbable  also  that  other  seeds 
germinated  later  in  the  season,  though  this  is  not  likely.  It  is  therefore 
safer  to  conclude  that  the  average  amount  of  growth  in  length  of  the 
epicotyledonary  stem  for  the  season  of  1908,  taking  all  seedlings  into  con- 
sideration, is  not  more  than  i  cm.  If  we  should  consider  only  those  which 
germinated  at  one  time,  at  the  beginning  of  the  growing-season,  this 
amount  would  probably  turn  out  to  be  somewhat  greater.  Under  the 
conditions  for  the  period  in  question  the  maximum  amount  of  growth 
was  5.5  cm.;  the  minimum,  1.5  mm.  Seedlings  of  these  dimensions,  and 
two  illustrating  the  average  growth  of  281  seedlings  (Station  2,  quadrat 
4) ,  are  reproduced  in  plate  1 7 ,  fig.  A.  Measurement  of  the  main  shoot  alone 
throws  out  of  account  the  growth  of  branches,  so  that  a  fuller  conception 
of  the  amount  of  development  possible  for  a  seedling  under  natural  con- 
ditions may  be  had  only  by  seeing  the  plants  themselves. 


Reproduction.  79 

RATE  OF  GROWTH  IN  MATURER  PLANTS   BEYOND  THE 
SEEDLING  STAGE. 

In  general  forestry  practice  the  use  of  formulas  is  directed  toward 
estimating  the  amount  of  lumber  in  the  trunk.  The  deduction  of  these 
formulas  is  easier  in  the  case  of  coniferous  trees  because  of  the  continuous 
growth  of  the  chief  shoot.  Special  problems  demand  formulae  based 
upon  other  data  than  the  rate  of  growth  of  wood,  e.g.,  in  the  business  of 
producing  cork  from  Quercus  suber.  When  forestry  practice  is  directed 
toward  the  culture  of  camphor  trees,  for  example,  in  which  the  whole 
bulk  of  the  plant  is  to  be  used,  the  desideratum  will  be  to  determine  the 
rate  of  increase  of  weight.  This  is  the  case  with  guayule,  since  the  whole 
of  the  plant  is  used  in  the  process  of  extraction  of  crude  rubber.  But 
the  rate  of  increase  in  weight  can  not  be  determined  without  introducing 
the  time  element,  so  that  we  must  first  determine  the  rate  of  stem  elonga- 
tion in  order  to  arrive  at  a  general  average  of  growth.  But  plants  of  the 
same  age  are  not  invariably,  or  even  quite  usually,  of  the  same  weight, 
since  the  relation  of  a  plant  to  its  environment  results  in  more  or  in  less 
bushiness,  in  partial  death  and  consequent  loss  of  branches,  in  unusually 
slow  or  rapid  growth,  or  in  total  loss  of  plant  by  death.  In  estimating 
the  weight  of  shrub  per  unit  of  area  for  some  future  time  it  is  evident  that 
all  these  factors  are  disturbing  elements,  the  values  of  which  may  not  be 
easily  determined.  About  the  best  we  can  do,  therefore,  is  (i)  to  determine 
the  average  rate  of  growth  in  length  of  stem,  and  (2)  to  determine  the  rate 
of  increase  in  weight  for  critical  periods.  The  data  indicate  that  there  is  a 
period  of  relatively  highest  growth-rate ,  expressed  in  stem  length  or  height , 
and  a  period  of  relatively  greatest  increase  in  total  weight  of  the  plant. 

RATE  OF  GROWTH  IN  TERMS  OF  STEM-LENGTH. 
It  has  already  been  shown  that  the  first  season's  growth  results  in 
an  average  stem-length  approximating  i  cm.  A  stem  of  this  size  has  no 
branches.  During  the  second  season's  growth  the  stem  may  simply 
lengthen,  or  it  may  also  produce  a  number  of  short  branches.  This  it 
is  more  certain  to  do  if  the  chief  shoot  produces  an  inflorescence.  It 
may  otherwise  merely  elongate  strictly  for  a  number  of  years,  resulting 
in  a  very  slow  increase  in  weight,  since  the  weight  is  affected  chiefly  by 
the  number  of  branches.  At  best  the  total  weight  assumed  by  a  plant 
in  the  first  7  to  10  years  is  small,  seldom  exceeding  a  few  ounces. 

RATE  OF  GROWTH  IN  EARLIER  YEARS  AFTER  GERMINATION. 

To  determine  precisely  the  age  of  a  given  seedling  is  more  difficult 
than  would  seem  at  first  glance  if  it  has  been  exposed  to  the  weather  for 
more  than  a  year.  Furthermore,  the  rate  of  growth  in  many  individuals 
is  so  slow  that  the  marks  become  well-nigh  effaced,  if  not  quite  so.  In 
obtaining  the  following  measurements,  only  plants  which  showed  the 
markings  plainly  enough  to  be  seen  clearly  have  been  used.  This  has 
very  naturally  thrown  out  those  of  very  slow  growth,  in  which  the  diffi- 
culties are  greatest,  and  thus  the  resulting  average  datum  is  probably 
too  great.  By  way  of  orientation  two  extreme  cases  may  be  cited.  One 
is  a  seedling  of  two  seasons'  growth,  which  germinated  in  1907,  making 


Guayule. 


in  that  year  3  cm.  and  in  the  following  year  n  cm.,  a  total  of  14  cm.  in 
the  two  years.  This  is  the  largest  field  plant  for  its  age  that  I  have  seen. 
In  contrast  is  cited  a  seedling  of  slow  growth,  fully  7  years  of  age,  entirely 
without  branches,  and  only  6  cm.  in  height.  The  average  rate  of  growth 
falls  between  these  extremes,  but  nearer  the  lower.  For  the  sake  of  brev- 
ity, as  it  would  serve  no  useful  purpose  to  introduce  large  tables  of  fig- 
ures, the  summaries  of  measurements  alone  are  given. 

The  average  rate  of  growth  of  30  seedlings  from  2  to  5  years  old  during 
particular  years  is  as  follows: 

TABLE  32. 


Age. 

1908. 

1907. 

1906. 

1905- 

1904. 

Average  amount  of  growth. 

mm. 

mm. 

mm. 

1  In  first  year,       1  7  mm. 

3 
4 

IS 
45 
Si 

16 

23 

3° 

20 

24 

15 

i  In  second  year,  20  mm. 
In  third  year,    37  mm. 
In  fourth  year,  5  1  mm. 

31  mm. 

The  average  amount  of  growth  in  seven  seedlings  for  the  last  three 
years,  1906-1908,  is  26  mm. 

Some  ten  seedlings  for  each  of  the  localities  mentioned  below  were 
measured,  giving  average  amount  of  growth  for  two  to  four  years,  as 
follows : 


Sierra  Candelaria 22 

Station  4 
Station  5 
Station  a 

Station  a  (Sierra  Zuluaga) 40 

Station  i  Qaguey) 30 


Sierra  Guadaloupe)  .  24 
Sierra  Guadaloupe)  .  18 
Sierra  Zuluaga) 31 


Cerritos  de  los  Calzones 20 

Cedros 34 

Apizolaya 42 

Lomerio  de  los  Zorrillos 49 

Encarnaci6n 26 

Average  rate  for  all 30 


It  will  be  seen  that  these  figures,  made  at  different  times  on  material 
from  different  localities,  check  each  other  fairly  well.  As  said  before,  the 
average  rate  of  growth  thus  deduced  is  probably  somewhat  high.  The 
rate  undoubtedly  increases  toward  the  fifth  year,  and  a  somewhat  more 
rapid  rate  is  then  maintained  during  a  few  years,  say  from  the  fourth  to 
the  seventh,  during  which  the  total  height  of  the  plant  increases  at  a 
greater  rate  than  before  or  after.  Usually  during  the  second  or  more  fre- 
quently the  third  year  a  set  of  branches  start  their  growth,  and  with  this 
the  weight  increases  more  rapidly.  What  this  weight  may  amount  to  in 
four  years  is  shown  by  3  thrifty  plants  taken  on  the  Lomerio  de  Zorrillos. 
These  made  growth  as  follows: 

TABLE  33. 


1905. 

1906. 

1907. 

1908. 

wSght. 

mm. 

IO 
20 
20 

mm. 

35 
5° 
40 

*6o' 
84 
1  60 

mm. 
IO 
40 

20 

grams. 

12 
30 
30 

Reproduction. 


81 


Hence  we  may  conclude  that  the  weight  gained  in  four  years'  growth 
can  scarcely  exceed  i  ounce,  and  probably  seldom  amounts  to  that. 

The  following  are  measurements  (in  millimeters)  from  rapidly  grow- 
ing plants  from  Station  2,  collected  in  January,  1908: 

TABLE  34. 


Plant. 

1902. 

1903. 

1904. 

1905. 

1906. 

1907. 

Notes. 

mm. 

mm. 

mm. 

mm. 

mm. 

'{ 

40 
40 

3° 
25 

94 
76 

32 
35 

40 
2O 

\  Average  height  from  base  of  1904 
/     growth,  2  lomm.  Weight  17  gms. 

2) 

16 

84 

65 

45 

\  Average    height    of    twigs    194 

2  I 

16 

80 

100 

5° 

j     mm.    Weight  12  gms. 

3 

•• 

•• 

17 

34 

67 

10 

Habit  strict,  with  short  branches 
above.    Weight  5  gms. 

4 

20 

30 

40 

70 

25 

25 

Not  less  than  6,  possibly  7,  years 
old.  Height  200  mm.  Dry  weight 

47  gms. 

It  is  of  interest  that  plant  4,  though  a  slower  grower  in  height  than 
i,  made  weight  about  twice  as  fast.  This  is  due  to  the  larger  number 
of  twigs.  Plant  4  may  be  regarded  as  an  expression  of  the  best  results 
which  may  be  expected  in  this  station.  We  may  therefore  conclude  that 
the  weight  of  4-year-old  plants  will  not  on  the  whole  exceed  1 5  grams  or 
0.5  ounce,  and  that  the  maximum  weight  for  a  6-year  plant  will  not 
exceed,  say,  45  grams  or  1.5  ounces. 


RATE  OF  GROWTH  IN  MEDIUM-SIZED  PLANTS. 

As  in  the  case  of  seedlings,  the  annual  accretions  of  growth  have 
been  measured  only  when  sufficiently  clear  for  certain  recognition.  The 
last  2  to  5  or  more  years'  growth  was  measured,  according  to  the  visi- 
bility of  the  markings.  Several  hundred  measurements  were  made  in 
all,  of  which  the  summaries  and  averages  alone  are  given  in  table  35. 

TABLE  35. — Average  amount  of  growth  per  year  in  the  localities  indicated. 


Locality. 

Average 
amount  of 
growth.    , 

Locality. 

—Average 
amount  of 
growth. 

Sierra  Candelaria  
Station  i  (Jaguey) 

mm. 

44 

Cedros  
Apizolaya                 

mm. 
56 
42 

2  (Sra.  Zuluaga)  
4  (Sra.  Guadaloupe). 

l\ 

Lomerio  de  los  Zorrillos  
Encarnaci6n  

49 
28 

5  (Sra.  Guadaloupe). 
Cerritos  de  los  Calzones  

38 

'la 

Caopas  
Average  of  all  

3i 
43 

1  NOTE. — The  plants  in  this  locality  showed  very  rapid  growth  in  1906,  explainable  by  the  rain  and 
by  their  having  been  previously  cropped  back.  The  branches  were  few  in  number,  so  that  the  plants, 
though  relatively  tall,  were  very  light  in  weight.  This  figure  would  therefore  better  be  thrown  out  of 
account,  in  which  case  the  average  falls  to  38  mm.  per  year.  The  datum  for  Station  2  has  been  checked 
up  by  a  later  count,  26  measurements  giving  an  average  of  32  mm.,  and  at  this  point  it  may  be  said 
that  the  data  above  given  are  collated  from  measurements  made  at  different  times,  results  being  used 
as  checks,  the  one  on  the  other. 


82  Guayule. 

In  addition  to  data  obtained  by  observation  of  external  marks,  a 
number  of  measurements  of  field  plants  were  made  by  the  usual  labora- 
tory method  of  marking  the  stem  with  India  ink.  The  results  of  these 
observations  are  here  given: 

Station  2,  quadrat  3...  6  twigs  marked  at  the  tip  with  a  drop  of  ink,  Jan.  6, 

1908.  Growth  commenced  Apr.  28.     Last  observation  Apr.  3,  1909.     Measure- 
ments as  follows,  in  mm.:    60,  75,  70,  75,  65,  50.     Average  amount  of  growth  for 
season,  66  mm. 

Station  i.     5  plants  marked  Jan.  3,  1908.     Last  observation  made  Apr.  3, 

1909.  The  mark  had  been  destroyed  on  2  plants.     The  total  amounts  of  growth 
for  the  3  remaining  were  30,  18,  and  35  mm.,  making  an  average  for  the  3  of  28  mm. 
All  growth  was  subsequent  to  May  29. 

Station  3.  Dec.  31,  1907.  3  marked  plants  showed  an  average  growth  of 
i  to  2  cm.  A  seedling  slightly  pruned  showed  2  cm.  new  growth  by  July  15.  The 
rate  of  growth  in  all  plants  at  this  station  was  small  in  1907. 

Station  6,  a  low  gravelly  ridge  in  the  playa,  Burrita.  4  plants  marked  Oct.  n, 
1907.  On  Jan.  u,  1908,  2  plants  showed  i  mm.  and  2  plants  2  mm.  growth  each. 
The  total  amount  of  growth  till  Aug.  21,  1908,  was  13,  20,  20,  and  10  mm.,  or  an 
average  amount  of  18  mm.  This  is  a  locality  of  conspicuously  slow  growth. 

The  average  amounts  of  growth  observed  in  marked  plants  for  the 
season  of  1907  were,  therefore,  66,  28,  20,  and  18  mm.,  making  a  grand 
average  of  growth  of  31  mm.  The  average  is  lower  than  the  one  above 
deduced  from  measurements  of  field  plants,  but  as  three  of  the  stations 
suffered  severely  from  drought  in  1907  the  rate  of  growth  was  probably 
rather  low.  Our  data  on  the  whole  indicate  that  the  rate  of  growth  of 
guayule  in  the  field  lies  somewhere  between  30  and  40  mm.  annually. 
This  general  conclusion  can  scarcely  be  said  to  be  too  optimistic.  It  will 
no  doubt  be  questioned  by  those  who  entertain  different  ideas  of  the 
rate  of  growth  of  this  plant.  The  belief  is  current  in  many  quarters  in 
Mexico  that  growth  is  much  more  rapid,  it  being  a  common  saying  that 
after  guayule  has  been  cut  the  crop  is  reestablished  in  five  years.  Such 
surprising  statements  were  made  to  me  regarding  one  locality  in  particu- 
lar that  I  made  special  effort  to  obtain  evidence.  Although  an  attempt 
to  visit  the  place,  some  leagues  to  the  west  of  Escalon  in  Chihuahua,  was 
frustrated,  I  succeeded,  through  the  courtesy  of  some  friends,  in  getting 
a  number  of  plants,  which,  though  of  somewhat  more  rapid  growth  than 
usual,  are  not  remarkable  in  a  special  degree.  The  plants  were  clean- 
limbed and  smooth-barked,  the  effect  of  this  more  rapid  growth.  They 
bear  evidence  of  a  heavier  rainfall  as  compared  with  plants  from  Zacate- 
cas,  but  this  appearance  is  due  in  part  to  the  fact  that  they  are  of  two 
different  types;  in  one  the  foliage  shoot  is  abruptly  terminated  at  the 
base  of  the  peduncle ;  in  the  other  the  shoots  taper  out  into  the  peduncle 
after  the  fashion  in  mariola.  The  branches  in  the  latter  are  thin,  die  back 
readily  and  often  for  a  good  distance,  and  in  these  plants  have  some  of 
the  characteristics  seen  in  the  stems  of  irrigated  plants.  I  give  measure- 
ments of  the  few  plants,  which  came  to  me  for  study,  in  detail  (table  36). 

The  average  amount  of  growth  of  each  plant  for  the  years  indicated 
is:  plant  i,  30  mm.;  plant  2,  41  mm.;  plant  3,  37  mm.;  and  for  all  the 
twigs  on  3  well-developed  plants  of  the  first -mentioned  type,  viz,  with 
abruptly  ending  foliage-shoots,  it  is  37  mm.  The  data  are  instructive  in 
that  they  point  to  a  "  fat "  year  preceding  two  "  lean  "  years,  namely,  1907 
and  1908.  The  rate  of  growth,  however,  compares  very  closely  with 
that  derived  from  material  from  other  localities. 


Reproduction. 
TABLE  36. 


Plant  No. 

Branch 
No. 

Amount  of  increase  in  stem  length  for  — 

1905. 

1906. 

1907. 

1908. 

mm. 

mm. 

mm. 

mm. 

I 

62 

41 

8 

8 

No.  i,  85  cm.  tall,  dry  weight  5  Ibs. 
5  oz.,  symmetrical,  well  developed. 

2 

3 

4 

45 

42 

57 

18 
10 
25 

10 
4 
18 

5 

26 

ii 

16 

Average 

u 

i 

117 

10 

20 

2 

86 

15 

3 

77 

28 

No.  2,  60  cm.  tall,  dry  weight  2.5  Ibs., 

4 

80 

22 

18 

irregularly  developed. 

5 

55 

16 

20 

6 

65 

10 

10 

7 

80 

23 

13 

8 

57 

8 

10 

Average  

76 

3  1 

17 

i 

5° 

60 

i? 

25 

No.  3,  35  cm.  tall,  dry  weight  9.5  oz.,  | 
10  yrs.  old,  well  developed,  symmet- 
rical. 

2 

3 
4 

5° 

II 

70 
62 
70 

20 
I? 

7 

10 

•i 

5 

25 

90 

15 

15 

Average  ' 

48 

70 

15 

17 

TABLE  37. 


Plant  No. 

Branch 
No. 

Amount  of  increase  in  stem-length 
for  — 

1906. 

1907. 

1908. 

No.  4,  seedling  23  cm.  tall.weight  10  gms. 
No.  5,  38  cm.  tall,  weight  48  gms  
Average 

mm. 
I 

mm. 

57 

mm. 
90 

mm. 
24 

(• 

60 


60 

48 
36 

45 

V 

90 

46 
70 

69 

No.  6 

i 

2 

3 
4 

•      5 
6 

7 
8 

9 

5° 
60 

65 

20 

3° 

80 

35 

47 

15 
3° 
23 
15 

12 

8 
15 

JL 

19 

£ 

40 
60 

6 
3° 
45 
3° 

JL 

44 

Average  

Average  for  all  years.    . 

37 

84 


Guayule. 


Table  37  gives  measurements  for  the  years  indicated  of  3  "  spindling  " 
plants,  which  grew  rapidly  in  height  but  did  not  develop  branches  and 
therefore  weight. 

Plant  7  was  50  cm.  tall  and  weighed  153  grams,  ragged,  but  showing 
abnormal  development  on  certain  shoots.  The  last  three  years  of  its 
growth  showed  accretions,  a  side-shoot  starting  low  down,  of  no,  200, 
and  60  mm.  The  upper  shoots  appeared  quite  similar  to  those  of  the 
other  plants,  but  were  more  or  less  damaged,  so  that  one  could  not  get 
satisfactory  measurements. 

The  conclusion  one  is  forced  to  draw  from  a  survey  of  the  above 
tables  is  that  in  a  certain  proportion  of  the  plants  in  the  locality  referred 
to  the  rate  of  growth  per  year  approaches  closely  to  50  mm.  In  these 
plants,  however,  the  branches  are  thin,  and  the  plants  are  not  well  devel- 
oped nor  heavy  for  their  size,  so  that,  economically  considered,  there  is 
nothing  gained.  Whether  the  differences  in  rate  of  growth  are  connected 
with  racial  differences  in  the  plants  is  discussed  elsewhere. 

RATE  OF  GROWTH  IN  IRRIGATED  PLANTS. 

A  considerable  number  of  plants  were  under  observation  for  the 
whole  of  two  growing-periods,  during  which  time  they  were  freely  irri- 
gated '  and  grew  rapidly,  at  a  rate  close  to  the  rhythmic  maximum.  The 
average  rate  of  growth  for  the  two  years  was  very  close  to  25  cm.  per 
year,  so  that  a  spread  of  a  meter  was  attained  by  nearly  all  of  the  plants. 
The  character  of  the  growth  is  described  elsewhere,  but  the  fact  here 
stated  indicates  very  clearly  that  plants  in  the  field  may  never  be  ex- 
pected to  reach  this  maximum.  The  greatest  growth  of  stem-length  in 
field  plants  for  one  year,  200  mm.,  was  seen  in  a  very  few  twigs  and  in 
shoots  favorably  placed,  the  rest  of  the  plant  failing  to  behave  similarly. 

The  weight  attained  in  two  seasons  by  irrigated  plants  growing 
from  small  butts  after  transplanting  is  upwards  of  2  pounds,  or  slightly 
over.  The  fresh  weight  of  a  large  plant  was  4.5  pounds.  Another  col- 
lected at  the  same  time  weighed  fresh  3.5  pounds,  and  shrank  in  drying 
to  i  pound  10  ounces.  The  dry  weight  of  two  others  was  28  and  32  ounces. 

On  the  other  hand,  plants  under  limited  irrigation  were  grown  at 
Caopas.  I  have  examined  three  sample  individuals  of  these,  a  large,  a 
medium-sized,  and  a  small  plant.  All  of  these  failed  to  start  promptly, 
and  had  been  pollarded.  The  amount  of  growth  made  by  them  is  shown 
in  table  38. 

TABLE  38. 


Size  of  plant. 

Distance 
pollarded 
above 
ground. 

Amount  of  growth. 

1908. 

1909. 

Total. 

Large  

cm. 

40 
3° 
15 

1  1 
16  to  18 
13 

cm. 
9 
10  to  12 
8 

cm. 

30 

25  to  30 

21 

Medium  

Small  

1  In  1907,  through  the  winter  until  the  following  April.  They  were  not  irri- 
gated later,  but  received  rain  in  the  summer.  They  had  a  sufficient  amount  of 
soil-moisture  for  continuous  growth. 


Reproduction.  85 

The  smaller  amount  of  growth  in  1909  was  due  to  the  absence  of 
irrigation,  as  elsewhere  explained.  It  will  be  noted  that  the  medium- 
sized  plant  responded  best,  which  in  general  comports  with  our  observa- 
tions of  the  rate  of  growth  of  field  plants. 

GENERAL  CONCLUSIONS. 

The  maximum  rate  of  growth  of  guayule  under  irrigation  is  in  the 
neighborhood  of  25  cm.  per  year  stem-length.  The  amount  of  growth 
between  the  field  average  and  the  maximum  average  for  irrigated  plants 
may  be  closely  regulated  by  irrigation,  to  which  the  plant  readily  responds. 

FIELD   PLANTS. 

When  it  is  borne  in  mind  that  the  total  height  of  a  plant  is,  except 
in  young  seedlings  of  strict  habit,  always  less  than  the  sum  of  its  longer 
annual  growths,  because  of  (a)  partial  dying  back  and  (6)  the  branching 
habit ;  and  when  also  it  is  remembered  that  numerous  plants  suffer  from 
untoward  conditions,  either  by  the  depredations  of  parasites  or  from 
poor  soil-conditions,  it  is  not  far  from  the  truth  to  say  that  the  average 
annual  rate  of  increase  in  height  is  3  cm.  A  plant  30  cm.  in  height  would 
therefore  be  10  years  old.  Plant  3,  above  described  (p.  83),  which  has 
undoubtedly  a  higher  rate  than  30  mm.  per  year,  is,  as  certainly  as  may 
be  estimated,  10  years  old.  As  has  been  said  previously,  however,  the 
important  desideratum  is  to  determine  the  period  of  life  during  which 
the  increase  in  weight  is  most  rapid,  aside,  of  course,  from  the  very  young 
seedling  stages,  when  the  ratio  of  increase  may  be  rapid,  but  the  total 
weight  very  little.  For  the  purpose  of  arriving  at  this  information,  I 
have  assumed  the  rate  3  cm.  per  annum  as  a  constant  factor.  A  large 
number  of  plants  have  been  weighed  and  measured,  and  the  data  thus 
derived  have  been  correlated  so  as  to  obtain  curves  of  increase  in  weight 
according  to  size  (fig.  13).  For  the  data  the  reader  is  referred  to  tables 
4  to  13. 

The  curves  have  not  been  constructed  for  plants  over  40  cm.  in  height 
for  two  reasons :  the  number  of  plants  beyond  this  size  is  very  much  smaller, 
and,  again,  their  age  is  too  great  to  admit  them  to  a  practical  considera- 
tion of  rotation  periods.  Observations  from  which,  in  part,  the  tables  of 
data  used  in  the  construction  of  the  curves  have  been  derived,  all  go  to 
show  that  the  first  pronounced  gain  in  weight  is  entered  upon  after  the 
plant  has  reached  a  height  of  30  cm.  The  average  weight  of  plants  of  this 
height  is  somewhat  over  5  ounces,  ranging  chiefly  between  2.5  and  8.5 
ounces.  The  average  weight  of  plants  40  cm.  tall  is,  on  the  other  hand, 
15  ounces  or  more.  That  is,  the  average  weight  is  trebled  in  making  the 
10  cm.  advance  in  height  beyond  30  cm.  This  is  shown  in  the  positions 
of  the  curves,  which,  however,  present  more  irregularities  than  one  would 
wish,  in  spite  of  the  fact  that  they  are  based  on  measurements  of  several 
hundred  plants.  The  greatest  fluctuations  in  the  curves  are  caused  by  the 
introduction  of  exceptional  individuals,  for  where  larger  numbers  are  used 
the  curves  are  more  uniform.  The  exceptional  individuals  may  be  either 
"spindling"  or  unusually  well-developed  in  point  of  ramification. 


Guayule. 


AGE  AND  HEIGHT. 

For  the  purpose  of  controlling  the  above  conclusion  I  have,  as  clearly 
as  possible,  made  estimates  of  the  ages  of  plants  of  various  sizes,  making 
no  assumption  as  to  the  rate  of  growth,  but  being  guided  solely  by  the 
marks  in  each  individual.  The  results  are  compiled  in  table  39. 

TABLE  39. — Size,  weight,  and  closely  estimated  ages  of  guayule  plants. 


Height 

Wei 

jht. 

ABC 

Fresh. 

Dry. 

Age. 

cm. 

oz. 

oz. 

yrs. 

I 

35 
3° 

iQ-75 
5  -o 

1  1 
8 

Very  well  developed,  symmetrical. 
Medium,  rather  undersized. 

28 

4-5 

7(8) 

Normal. 

25 

4-o 

7 

Do. 

17 

0-875 

6 

Do. 

II 

51 

18 

ii.  6 

14 

Y-shaped,  symmetrical.     "Macho." 

35 

8 

4-0 

10 

Do. 

III 

5° 

12 

6.7 

10  tO   12 

Y-shaped,  symmetrical.    "  Hembra." 

3° 

6 

3  -25 

7  to  8 

Do. 

IV 

66 

32 

24.0 

19   tO  20 

Y-shaped,  narrow-leafed  type. 

40 

7 

4-5 

10  tO   12 

Do. 

V 

40 
35 

10 

5 

6.0 
3.06 

i5(?) 

12 

Slow-growing,  broad-leafed  type. 

VI 

5° 

54 

36 

17    tO  20 

Densely  branched,  spreading,  in  full 

leaf. 

45 

44 

29 

16  to  18 

(NOTE.     Plants  of  series  VI  are  the 

33 

21 
20 

16 
6 

5 

14 

5 
3-75 

10  tO   II 

9 
8 

heaviest  for  their  height  of  any, 
except  very  occasional  plants  such 
as  that  in  plate  8,  fig.  B.) 

IS 

1.75 

0-93 

6 

VII 

65 

64 

43 

20 

V-shaped,  half-spreading  type,  dense- 
ly branched,  symmetrical  (plate  8, 
%.  B). 

55 

32 

18 

15  to  1  6 

5° 

18 

15 

13  to  15 

33 

8 

5-5 

9 

28 

6 

4 

8 

24 

3 

i   5 

7 

22 

i-2S 

0   5 

6 

VIII 

55 

18.5 

15  to  16 

Y-shaped  (plate  8,  fig.  A). 

30 

6 

ii 

Normal  shape  for  age. 

25 

i 

9 

Do. 

20 

0.875 

7 

Do. 

14 

°-5 

5 

Do. 

4-5 

0.  I 

2 

Do. 

These  data  have  been  charted  in  the  accompanying  curves,  corre- 
lating age  and  height  (fig.  15,  upper  diagram).  It  is  an  obvious  objection 
to  the  value  of  these  curves  that  they  are  based,  necessarily,  on  compara- 
tively few  plants,  but  their  value  is  enhanced  by  the  individual  treat- 
ment, since  the  estimate  of  age  was  made  with  great  care.  A  fairly  close 
correlation  emerges,  however,  from  the  diagram,  from  which  we  see  that 
plants  of  10  years  of  age  have  a  height  of  about  30  cm.,  and  those  of  15 
years  about  40  cm. 

An  increase  in  height  of  10  cm.  over  30  or  35  cm.  is  correlated  (judg- 
ing from  the  data  of  table  39)  with  a  doubling  at  least  of  the  weight 


Reproduction. 


87 


HEIGHT   IN    CENTIMETERS 


FIG.  13. — Curves  correlating  height  and  weight  in  the  plants  recorded  in  tables  5  to  7  and  9  to  13,  inch 
sive.     The  approximate  averages  are  indicated  in  the  curve  of  averages. 


s 

I" 

z 

I" 
*   .„ 


~L 


FIG.  14. — Curves  correlating  height  and  weight  of  the  plants  in  table  39. 


88 


Guayule. 


12          13          14  13 


ACC   IN   YEARS 


zo 

U    18 


AGE  IN  YEARS 


FIG.  15. — Upper  diagram:  Curves  correlating  age  and  height  of  plants  in  table  39.     Lower  diagram: 
Age  and  weight  correlated.     The  same  plants. 


Reproduction.  89 

(fig.  14).  Fig.  13,  on  the  other  hand,  indicates  a  greater  increase,  to 
nearly  three  times  the  weight  at  30  cm.  The  average  weight  of  a  plant 
30  cm.  in  height  is,  according  to  fig.  14,  about  7  ounces,  but  as  the  plants 
considered  in  this  curve  are  normally  developed  or  indeed  considerably 
above  the  average,  the  average  weight  of  a  30  cm.  plant  is  probably 
nearer  to  that  indicated  in  fig.  13,  viz,  5  ounces.  The  mere  ratio  of 
change  in  weight  is  not  peculiar  to  these  dimensions  alone.  What  appears 
from  the  data  is  that  the  weight  of  plants  up  to  the  height  of  30  cm.  is 
not  great  enough  for  economical  harvesting.  The  increase  in  size  and 
weight,  however,  is  as  great  in  the  subsequent  five  years  as  in  the  previous 
ten,  so  that  taking  the  crop  at  the  end  of  ten  years  would  give  results 
only  half  as  great  as  the  returns  of  a  fifteen-year  rotation  period. 

This  conclusion  is  shown  graphically  by  fig.  15,  lower  diagram,  which 
indicates  that  the  weight  of  a  plant  advances  from  about  6  ounces  at 
ten  years  of  age  to  1 5  ounces  at  fifteen  years.  A  considerable  minus  error 
in  the  estimation  of  ages  might  be  allowed,  and  yet  the  increase  indicated 
in  the  preceding  paragraph  would  still  be  shown.  It  is  fair  to  state,  how- 
ever, that  there  is  little  chance  for  such  error,  as  I  have  taken  the  pre- 
caution of  being  conservative  when  there  was  doubt. 

Fig.  15,  upper  diagram,  indicates  that  the  estimate  of  rate  of  growth 
used  throughout,  viz,  3  cm.  per  year,  is  very  nearly  correct.  We  may 
therefore  conclude: 

(1)  That  the  average  rate  of  growth  of  guayule  per  annum  is  about 
3  cm. 

(2)  That  the  amount  of  increase  in  weight  between  the  tenth  and 
fifteenth  years  of  its  age  is  at  least  as  great  as  that  occurring  during  the 
first  ten  years;  and  that  this  further  justifies,  from  an  economic  point 
of  view,  a  fifteen  rather  than  a  ten  year  rotation  period,  aside  from  con- 
siderations which  might  be  drawn  from  loss  by  death  (p.  71),  could  we 
ascertain  this  accurately  enough. 


CHAPTER  V. 
ANATOMY  AND  HISTOLOGY. 

While  the  anatomy  of  the  Composite  has  been  studied  in  much 
detail,  beginning  with  von  Sachs,  followed  by  van  Tieghem,  Vesque, 
Vuillemin,  Col,  and  less  voluminously  by  other  writers,  that  of  the  genus 
Parthenium  had,  up  to  1901,  received  no  examination.  In  that  year  the 
plant  which  supplies  the  object  of  the  present  treatise  came  to  the  atten- 
tion of  the  French  botanists  MM.  Fron  et  Francois  (1901),  who  gave  a 
brief  account  of  the  more  obvious  features  of  the  anatomy  of  the  stem 
and  of  the  structure  of  the  fruit.  A  more  extensive  paper  was  published 
in  1908  by  Dr.  H.  Ross,  who  visited  Mexico  in  1907  and  examined  the 
guayule  in  the  field,  chiefly  about  Saltillo.  In  this  paper  an  anatomical 
study  of  guayule  was  supplemented  by  brief  reference  to  two  other  species, 
P.  incanum  and  P.  tomentosum.  To  both  these  contributions,  as  also  to 
those  of  more  general  import,  reference  will  presently  be  made. 

ROOT. 

PRIMARY  STRUCTURE. 

The  primary  root  is  diarch.  The  two  bundles  of  protohadrome,  of 
spiral  vessels,  become  early  united  by  a  centripetal  development  of  vessels 
forming  a  primary  plate,  on  either  side  of  which  stand  the  two  protolep- 
tome  strands.  At  this  time  the  stele  has  a  continuous  pericambium  and 
is  surrounded  by  a  well-marked  endodermis,  which  may  be  recognized  by 
the  bands  of  Caspary  and  by  the  starch -content  of  the  cells  (plate  22,  figs. 
6-8).  The  starch-grains  are  relatively  large  and  are  compound.  Their 
persistence  is  variable,  traces  being  visible  for  some  months  in  some 
instances,  e.g.,  in  a  root  4  mm.  in  diameter;  in  other  cases  they  may  have 
disappeared  in  a  few  weeks.  Thus  in  a  root  0.46  mm.  in  diameter,  in 
which  radial  thickening  of  the  endodermis  had  just  commenced,  starch  in 
these  cells  fluctuates,  there  being  now  more  and  now  less,  apparently 
according  to  the  draft  upon  it  by  the  tissues.  Without  the  endodermis 
lie  three  layers  of  cortical  cells  with  extensive  intercellular  spaces,  which, 
however,  do  not  occur  between  the  outer  layer  of  cortical  cells  (the  hypo- 
dermis)  and  the  epidermis. 

SECONDARY  STRUCTURE. 

The  epidermis  begins  very  early  to  break  down,  so  that  in  a  root  less 
than  0.5  mm.  in  diameter  the  earliest  peridermal  divisions  have  set  in. 
These  do  not  usually  occur  in  the  outermost  cortical  cells,  which  here 
take  on,  in  a  weak  fashion,  the  characters  of  an  exodermis,  as  described 
for  Cephalanthus  and  Tecoma  by  Holm  (1907),  but  in  the  second  hypo- 
dermal  layer  (plate  22,  fig.  7).  At  this  time  growth  commences  in  the 
cortex,  both  radial  and  periclinal  divisions  occurring  (plate  22,  fig.  8). 
Growth  of  the  endodermis  is  concurrent  (plate  2  2 ,  fig.  6) .  Both  radial  and 
90 


Anatomy  and  Histology.  91 

tangential  increase  in  size  results  in  (i)  extension  of  the  radial  dimensions 
of  the  so-placed  walls.  This  extension  is  confined  to  that  part  of  the  wall 
between  its  outer  limit  and  the  band  of  Caspary,  leaving  this  band  in  the 
same  position  as  before  (plate  22,  figs.  6,  8).  With  this  fact  in  mind  the 
endodermis  may  be  identified  for  a  long  time,  indeed  frequently  till  it 
is  well-nigh  expelled  by  secondary  thickening.  (2)  Tangential  growth  is 
accompanied  by  cell-divisions  in  the  radial  direction,  Casparian  bands 
being  formed  in  the  new  walls  (plate  22,  fig.  6).  In  particular  positions, 
namely,  opposite  the  leptome  bundles,  the  earliest1  resin-canals  appear.2 
These  do  not  belong  to  the  primary  structure  of  the  root,  but  arise  second- 
arily in  the  endodermis  (plate  22,  figs,  i  to  5).  Their  mode  of  development 
is  as  follows:  3  or  4  adjacent  cells  divide  by  periclinal  walls,  thus  bringing 
it  about  that  two  or  three  places  occur  where  4  cells  lie  with  their  angles 
adjacent.  Here  the  walls  split  apart,  making  a  simple,  prismatic,  inter- 
cellular space  without  demonstrably  different  contents;  the  adjacent  cells 
divide  radially,  so  that  each  canal  has  now  4  cells  contingent  upon  and 
peculiar  to  it.  Later,  further  divisions,  roughly  parallel  to  the  early  canal- 
walls,  result  in  the  canal  consisting  structurally  of  two  layers  of  cells 
constituting  a  prismatic  tube.  It  is  seen  that  the  endodermal  canals,  for 
so  they  will  be  called,  are  arranged  in  two  groups,  of  usually  two  or  often 
three  or  occasionally  even  four  canals  each,  each  group  being  placed  oppo- 
site a  primary  leptome  bundle.  This  relation  was  first  described  by  von 
Sachs  for  Helianthus.  The  structure  of  these  canals  does  not  change, 
though  there  ensues  some  displacement  of  the  cells  in  roots  which  have 
thickened  abnormally  without  losing  the  outer  primary  tissues.  The  figures 
displaying  the  cell  lineage  will  make  this  clear  (plate  22,  figs,  i  to  5). 

The  growth  of  the  endodermis  may  be  followed  till  it  is  thrown  off 
by  the  formation  of  cork  within  it.  During  its  history  it  enlarges  from 
a  cylinder,  of  o.i  mm.  inside  diameter,  of  18  to  20  cells,  to  one  of  3  mm. 
diameter,  composed  of  hundreds  of  cells,  or  even  to  larger  dimensions, 
before  being  finally  cut  out.  Throughout  the  greater  portion  only  radial 
divisions  occur,  though  the  cells  increase  in  radial  depth.  In  the  region 
of  the  canals,  however,  the  endodermal  cells  divide  in  a  general  periclinal 
direction,  giving  rise  to  two  or  even  more  irregular  series  of  cells. 

In  the  ultimate  condition  of  the  endodermis  and  of  the  secondary 
cortex  the  walls  of  the  cells  are  reticulately  thickened  (plate  22,  fig.  15), 
so  that  in  a  root  2  mm.  in  diameter,  of  a  field  seedling,  the  endodermis 
may  be  followed  all  the  way  around  with  great  ease,  provided  that  the 
rubber  has  been  previously  extracted.  The  reticulations  are  the  result 
of  the  oval  form  of  the  broad,  shallow  pores,  which  are  somewhat  crowded. 
They  are  more  strongly  developed  in  plants  grown  under  normal  condi- 
tions, as  appears  from  the  fact  that  in  an  irrigated  seedling,  with  a  root 

1  Ross  (1908,  p.  25),  in  stating  that  there  are  only  a  few  canals  in  the  primary 
cortex  of  the  root,  does  not  make  it  clear  that  he  refers  to  canals  of  endodermal 
origin. 

2  Exceptions  occasionally  occur  in  which  the  canals  in  one  half-circle  of  the 
hypocotyl  do  not  approach  on  entering  the  root,  and  conversely,  cases  occur  in 
which  the  grouping  of  the  canals  occurs  in  the  hypocotyl,  on  one  side  of  it.     In 
other  words,  the  root-structure  is  taken  on  at  a  higher  level  on  one  side  than  on 
the  other. 


92  Guayule. 

4  mm.  in  diameter,  the  reticulations  were  much  less  marked  or  absent. 
The  same  condition  is  found  in  the  definitive  stem. 

This  large  size  of  the  cylinder  of  tissue  inclosed  within  the  endoder- 
mis  is  attained  only  under  an  abundant  water-supply  and  other  condi- 
tions insuring  rapid  growth.  Under  such  circumstances  the  structure  of 
the  canal  itself  passes  beyond  the  normal  definitive  stage,  and  the  cells, 
usually  and  normally  eight  in  number  in  transverse  section,  may  suffer 
further  divisions  as  shown  in  plate  22,  fig.  5,  to  an  extent  sufficient,  to- 
gether with  some  displacement,  to  render  it  somewhat  difficult  to  exactly 
delimit  the  structure.  In  a  root  of  this  size,  viz,  4  mm.  in  diameter,  the 
endodermis  may  still  be  readily  recognizable  (though  irregular  in  charac- 
ter, being  in  part  of  two  rows  of  cells)  by  the  starch-content  or  by  the 
Casparian  spots,  or  both.  The  position  of  the  endodermis  is  always  clearly 
shown,  other  signs  failing,  by  the  primary  canals. 

The  physiological  changes  in  the  endodermis  are  of  particular  inter- 
est. Reference  has  been  made  to  the  variableness  of  the  starch-content 
of  its  component  cells.  When  grown  under  irrigation  the  starch  may  be 
seen  in  much  larger  plants  than  in  those  which  have  grown  under  normal 
or  field  conditions.  In  these  growth  is  less  rapid  and  the  extension  of  the 
tissues  correspondingly  less  marked.  In  these  also  the  secretion  of  rubber 
ensues  earlier  and  is  correlated  with  the  occurrence  of  drought.  In  such 
plants  the  cells  of  the  endodermis,  together  with  others  to  be  noted  below, 
secrete  rubber,  so  that,  in  a  small  seedling  with  a  tap-root  2  mm.  in  diam- 
eter, the  cells  of  the  endodermis  will  be  found  engorged  with  this  sub- 
stance. The  cells  of  the  canals  are  especially  noteworthy  in  this  respect. 
By  taking  advantage  of  the  effect  of  water  upon  the  rate  of  secretion,  it 
may  be  shown  that  the  secretion  of  rubber  in  the  endodermis  takes  place 
first  in  the  cells  of  the  resin-canals  (plate  41,  fig.  6).  Thus,  in  the  root, 
4  mm.  in  diameter,  of  a  seedling  which  grew  with  great  rapidity,  the  canal 
calls  were  half-filled  with  small  droplets  of  secretion  which  reacted  to  al- 
kanet  The  specimen  had  previously  been  freed  from  alcohol-soluble  sub- 
stances, and  there  can,  I  think,  be  no  doubt  of  the  nature  of  the  material 
in  question.1 

The  behavior  of  the  pericambium  in  the  region  included  between  the 
primary  leptome  and  the  endodermis  differs  from  its  behavior  elsewhere. 
One  finds,  in  a  root  1.2  mm.  in  diameter,  that  the  pericambial  cells  have 
enlarged  radially  and  have  in  some  cases  undergone  periclinal  divisions 
and  the  daughter-cells  further  radial  divisions  (plate  23,  fig.  4).  The  peri- 
clinal divisions  suggest  initial  cork-divisions,  but  this  is  not  the  case,  as 
both  the  radial  divisions  and  the  further  behavior  of  the  cells  show.  With 
a  slight  increase  in  thickening  in  the  root,  sufficient  to  attain  1.5  mm.  in 
diameter,  the  cell-walls  are  a  little  thickened  and  a  rearrangement  has 
taken  place.  The  cells  have  apparently  been  compressed  between  the  pri- 
mary leptome  and  the  endodermis,  and,  under  suitable  conditions,  as  in 
the  specimen  from  which  plate  22,  fig.  5,  was  made,  have  secreted  rubber, 

1  I  have  noticed  that  reaction  to  alkanet,  which  is  the  same  in  all  the  cells 
at  first,  becomes  in  the  canal-cells  darker  with  time,  the  preparation  having  been 
kept  in  darkness.  I  have  attributed  this,  with  some  doubt,  to  the  greater  proto- 
plasmic content  of  these  cells.  Great  care  was  taken  to  extract  very  thoroughly 
with  absolute  alcohol,  and,  in  a  part  of  my  preparations,  with  caustic  potash  also. 


Anatomy  and  Histology.  93 

though  less  than  in  endodermal  cells  with  which  they  are  in  immediate 
contact.  Further  development  sees  the  collapse  of  the  pericambium  cells 
(plate  22,  fig.  6),  and,  as  seen  elsewhere,  the  primary  stereome  occurs  in 
the  primary  leptome  just  within  the  pericambium. 

The  primary  cortical  cells  outside  of  the  endodermis  are  also  capable 
of  secreting  rubber.  That  they  do  so  at  all  is  contingent  on  the  rate  of 
growth  of  the  seedling.  If  this  is  rapid  enough  to  remove  the  cortex 
before  drought  sets  in,  no  appreciable  secretion  will  have  occurred.  If, 
however,  the  rate  of  growth  is  lower,  so  that  for  the  greater  part  of  a  year 
the  tissue  in  question  remains  functional,  the  inner  cells  at  least  may  be 
found  densely  filled  with  rubber.  In  the  root,  5  mm.  in  diameter,  of  a 
field  seedling  fully  a  year  old,  the  following  measurements  (along  a  radius) 
were  made,  from  which  an  idea  of  the  amount  of  primary  cortex  remaining 
active  may  be  had:  Wood,  1.4  mm.;  secondary  cortex,  0.64  mm.;  pri- 
mary cortex,  0.15  mm.;  cork,  0.27  mm. 

EARLY  SECONDARY  CHANGES  IN  THE  STELE:  (HADROME). 

With  the  completion  of  the  primary  hadrome  plate  there  ensues  a 
centrifugal  development  of  this  tissue  by  the  direct  transformation  of  the 
protogenic  cells  adjacent  to  the  middle  part  of  the  plate.  The  increase  of 
hadrome  extends  along  all  radii  except  those  lying  near  the  plane  of  the 
primary  plate,  but  usually  rather  less  rapidly  toward  the  primary  lep- 
tome, so  that  in  transverse  section  there  appear  two  wings,  so  to  speak, 
of  hadrome.  This  is  protogenic,  but  is  added  to  quite  soon  by  the  activity 
of  a  cambium  which  first  becomes  apparent  within  and  close  to  the  pri- 
mary leptome  bundles  (plate  22,  fig.  10),  and  extends  toward  and  finally 
around  the  outer  edges  of  the  primary  hadrome  (plate  22,  figs.  9  to  10). 

Up  to  this  point  in  the  development  of  the  stele  nothing  exceptional 
is  seen.  The  only  question  which  has  been  raised  is  in  regard  to  the  pre- 
cise origin  1  of  the  earlier  formed  secondary  hadrome  elements,  whether 
this  is  by  means  of  the  cambium  which  arises  on  the  inner  surface  of  the 
primary  phloem,  or  is  directly  from  protogenic  elements  lying  adjacent  to 
the  primary  hadrome  plate.  The  evidence  from  the  material  here  under 
discussion  is  that  the  latter  is  the  case. 

Now,  however,  a  behavior  ensues  which  is  somewhat  unusual.  Two 
independent  mestome  strands  of  (at  first)  a  single  radial  series  of  vessels 
and  a  very  small  leptome  strand  arise,  each,  usually,  in  immediate  con- 
tact with  the  primary  trachea  (plate  22,  figs.  9  to  1 1).  The  emergence  of 
a  secondary  root  disturbs  the  exact  position  so  that  the  earliest  vessels 
may  be  somewhat  removed  from  the  primary  hadrome.  A  similar  condi- 
tion has  been  observed  by  me  in  Lamium  amplexicaule ,  and  by  Petersen  2 
in  other  Labiatae. 

It  is  necessary  to  note  that  although  these  mestome  bundles  are  ver- 
tically below  the  two  lateral  cotyledonary  traces,  we  shall  presently  see 
that  they  are  independent  of  these  and  have  no  connection  whatever  with 
them. 


I  De  Bary,  Comparative  Anatomy  of  the  Phanerogams. 

I 1  have  not  seen  this  paper,  but  am  informed  by  Dr.  Holm. 


94  Guayule. 

The  appearance  of  the  stele  is  now  suggestive  of  a  tetrarch  structure 
and  is  as  follows:  In  a  plane  at  right  angles  to  the  primary  hadrome 
plate  lie  the  two  leptome  bundles,  between  which  and  the  hadrome  plate 
the  primary  cambium  lies.  In  a  plane  coincident  with  the  hadrome  plate 
and  just  beyond  its  edges  lie  two  small  secondary  mestome  strands, 
formed  independently.  These,  which  I  shall  here  call  the  intercalated 
strands,  lie,  therefore,  in  a  plane  between  the  two  broad  primary  medul- 
lary rays  (plate  22,  fig.  10).  The  isolated  condition  of  the  intercalated 
bundles  is,  however,  very  transient,  since  the  parenchyma  rays  between 
the  axial  hadrome  strand  and  the  small  intercalated  bundles  are  soon 
bridged  over,  the  whole,  save  as  mentioned  in  the  following  paragraph, 
coalescing  to  form  a  single  axial  strand  of  hadrome.  Additional  second- 
ary bundles  are  intercalated  between  those  already  present,  at  first  from 
the  four  angles  of  the  hadrome  wings,  so  that  in  a  tap-root  1.2  mm.  in 
diameter  before  me  (plate  40,  fig.  i)  there  appear  8  bundles,  though  it 
must  be  said  that  the  appearance  of  the  stele  in  roots  of  the  same  size  is 
not  by  any  means  uniform. 

The  closure  of  the  hadrome  wings  by  meeting  the  xylem  of  the  inter- 
calated strands  is  not  complete,  and  thus  are  left  two  islands  (analogous 
to  medullary  spots)  of  unlignified  cells  about  the  edges  of  the  primary  ha- 
drome plate  (plate  22,  fig.  n).  The  outlines  of  these  islands  are  quite 
irregular,  and  they  may  ultimately  become  compressed  or  lignified,  so 
that  it  is  only  with  difficulty  that  they  may  be  recognized.  In  thin  roots, 
as  especially  in  the  fibrous  laterals,  the  wood  cylinder  is  very  compact, 
and  may  have  no  parenchyma  rays.  In  such  also  the  secondary  changes 
in  the  cortex  are  less  extensive,  and  the  pericycle  is  much  compressed. 

LATER  SECONDARY  CHANGES:  (CORTEX). 

With  age  the  walls  of  the  cortical  cells  become  somewhat  thickened 
and  pitted.  The  intercellular  spaces  are  very  regular  in  shape,  and  regu- 
larly disposed.  In  a  tangential  section  they  appear  very  uniformly  len- 
ticular (plate  28,  fig.  5). 

STEREOME  AND  SECONDARY  CANALS. 

Aside  from  the  secondary  increase  of  wood  and  bast,  the  appearance 
of  stereome  and  of  secondary  canals  has  to  be  mentioned.  That  stereome 
which  appears  in  connection  with  primary  tissues  only  may  properly  be 
spoken  of  as  primary.  Of  this  there  are  but  two  slender  bundles  (plate 
23,  fig.  6),  which  consist  each  of  a  few  (less  than  a  dozen)  slender,  very 
thick-walled  elements  buried  in  a  mass  of  material  derived  from  the  pri- 
mary leptome  by  the  swelling  of  the  cell-walls  till  the  lumina  become 
indistinguishable. 

The  method  of  origin  of  these  sclerenchyma  cells  is  difficult  to  deter- 
mine, and  will  be  discussed  beyond. 

The  pericambium  appears  to  be  continuous,  but  so  far  as  I  am  aware 
the  formation  of  stereome  does  not  involve  the  cells  of  this  layer.  The 
configuration  of  the  cells  of  the  adjacent  secondary  cortex  is  at  this  time 
(a  root  2  mm.  in  diameter)  very  curious,  the  walls  having  been  distorted, 
as  if  by  stretching  in  a  radial  direction  from  the  primary  sclerenchyma  as 
a  center.  This  result  would  seem,  however,  to  be  due  to  compression  by 


Anatomy  and  Histology.  95 

the  growth  of  a  mass  of  secondary  stereome  which  arises  within  the  pri- 
mary stereome  and  is  removed  from  it,  in  a  root  of  2  mm.  diameter  (plate 
23,  fig.  6),  by  about  35  microns  toward  the  center. 

This  secondary  stereome  strand  is  larger  than  the  primary  strand 
and  becomes  an  obvious  structural  feature.  For  this  reason,  and  on  ac- 
count of  its  close  proximity  and  its  relative  position  to  the  primary  canals, 
it  may  very  easily  be  mistaken  for  the  primary  strand.  Its  cells,  however, 
are  larger,  and  it  arises  in  connection  with  secondary  leptome,  and  not  in 
relation  to  the  proto-leptome.  For  this  reason  its  position  is  more  variable 
than  that  of  the  primary  strand  and  may  suffer  tangential  displacement 
(due  to  unequal  development  of  the  root),  as  shown  in  plate  23,  fig.i ;  and 
further,  for  less  obvious  reasons,  the  stereome  may  not  occur  at  all. 

Other  secondary  stereome  strands  develop,  if  at  all,  always  in  con- 
nection with  the  leptome,  as  stated  for  the  stem  by  Fron  and  Francois 
(1901)  and  by  Ross  (1908).  The  particular  mode  of  origin  will  be  discussed 
later.  Each  series  is  circular  (plate  23 ,  figs,  i  and  2) ,  as  all  the  members  of 
a  series  arise  normally  at  the  same  time,  though  the  series  may  be  more  or 
less  discontinuous,  owing  to  unequal  development  as  between  the  mem- 
bers of  the  series.  In  seedlings  grown  rapidly  under  irrigation  the  amount 
of  stereome  development  is  usually  notably  less  than  in  field  seedlings  or 
in  others  grown  slowly. 

Secondary  resin-canals  (plate  22,  fig.  13)  arise  within  the  secondary 
leptome  in  close  proximity  to  the  cambium,  and  in  the  manner  described 
by  Ross  for  the  canals  of  the  stem.  They  consist  at  first  of  two  tangential 
rows  of  cells  scarcely  distinguishable  from  the  cambium  from  which  they 
arose,  though  quite  early  they  may  be  recognized  by  their  larger  size  and 
the  dense  protoplasmic  contents  which  at  first  show  by  their  reactions 
merely  their  protoplasmic  nature.  Ross  (1908,  p.  260),  however,  says 
of  these  canal-cells:  "Die  den  Kanal  auskleidenden  Zellen  sind  durch 
dichtes  Protoplasma  ausgezeichnet,  das  sich  mit  Chlorzinkiod  dunkel- 
braungelb,  mit  Alkannin  intensiv  rot  farbt,  wahrend  sonst  das  Leptom 
hauptsachlich  starkereichen  Zellinhalt  fuhrt." 

My  own  observations  differ  from  those  of  Ross  in  that  the  cell  con- 
tents, when  very  young,  do  not  react  to  alkanet  as  described  by  him. 
Very  soon  after  the  two  cell-rows  begin  to  split  away,  minute  globules  of 
a  secretion  begin  to  appear,  and  these  indeed  take  on  the  intensive  red 
color  of  the  reagent.  This  is  considerably  in  advance  of  the  same  appear- 
ances in  the  adjacent  cortical  cells.  Preparations  treated  with  alcohol  to 
dissolve  out  the  resin  or  oil,  which  might  be  said  to  occur,  show  this  very 
clearly,  and  further  treatment  of  the  same  preparations  with  benzole 
shows  that  these  intensively  staining  masses  are  dissolved  out  by  that 
agent,  and,  in  the  absence  of  evidence  to  the  contrary,  must  be  regarded 
as  rubber.  In  the  secondary  canal-cells,  therefore,  as  in  the  wall-cells  of 
the  primary  canals  of  the  root,  occurs  the  earliest  appearance  of  rubber, 
the  secretive  activity  extending  progressively  from  them  to  the  surround- 
ing tissues,  and  more  rapidly  in  the  primary  cortex.  It  is  worthy  of 
especial  note  that  rubber  occurs  in  the  wall-cells  of  canals  which  normally 
contain,  in  the  meatus,  the  resin  characteristic  of  the  guayule  plant.  This 
point  calls  for  discussion,  which  will  follow  later. 


96  Guayule. 

The  leptome  of  the  root  does  not  show  any  starch-content  in  the 
sieve  part,  though  it  occurs  apparently  erratically  in  its  parenchyma  and 
in  the  cortical  cells  adjacent  also  to  the  leptome  and  to  the  resin-canals. 
It  is  also  to  be  found  in  the  endodermal  cells  close  to  the  primary  canals, 
and  occasionally  elsewhere,  in  a  4  mm.  diameter  root  of  a  field  plant. 

If,  however,  we  examine  a  plant  grown  with  an  abundance  of  water, 
in  which  the  secretion  of  rubber  has  taken  place  only  in  minute  quantities 
and  this  in  the  wall-cells  of  the  resin-canals,  an  important  physiological 
relation  between  starch  and  the  secretory  canal-cells  is  indicated.  In  a 
root  4  mm.  in  diameter,  which  developed  in  about  three  months,  the  dis- 
tribution of  starch  and  its  quantity  are  very  striking.  It  is  present  abun- 
dantly (a)  in  a  broad,  irregular  radial  band  of  cortical  cells  extending 
from  the  primary  resin-canals,  (6)  in  a  narrow  and  somewhat  irregular 
circular  band  midway  the  secondary  cortex,  and  (c)  in  marked  quantities 
in  the  cortical  cells  adjacent  to  the  definitive  resin-canals.  It  is  not  pres- 
ent in  the  leptome  adjacent  to  the  young  resin-canals.  It  would  therefore 
seem  probable  that  the  presence  of  starch  in  marked  quantities  near  the 
resin-canals  is  related  either  to  the  secretion  of  rubber  by  the  wall-cells 
especially,  or  to  the  secretion  of  resin.  The  familiar  case  of  Pinus,  in 
which  starch  occurs  near  the  resin-canals,  suggests  the  latter. 

The  earliest  appearance  of  rubber,  which  is  secreted  by  the  paren- 
chyma of  the  cortical  rays  and  of  the  cortex,  aside  from  the  cells  of  the 
resin-canals  as  above  noted,  is  to  be  seen  in  the  innermost  cells  of  the  rays, 
and  synchronously  in  the  outermost  cells  of  the  primary  cortex,  or,  if  that 
is  absent  before  secretion  begins,  of  the  secondary  cortex.  This  fact  is 
beautifully  shown  in  the  tap-root  of  a  seedling  from  the  field,  probably 
less  than  one  year  old,  collected  on  July  14,  1908,  and  measuring  2  mm. 
in  diameter.  In  this  specimen  the  cells  of  the  primary  cortex  were  com- 
pletely filled,  as  also  the  outer  cells  of  the  secondary  cortex,  there  being 
progressively  less  and  less  secretion  toward  the  center  of  the  root.  The 
opposite  relation  was  shown  by  the  parenchyma  rays,  in  the  cells  of  which 
the  amounts  of  rubber  were  found  to  be  progressively  less  and  less,  as  one 
proceeded  from  the  center  outward  (plate  23,  figs.  3,  7;  plate  40,  figs.  2  to 
4).  In  a  still  younger  seedling,  perhaps  three  months  old,  about  1.2  mm. 
in  diameter,  rubber  is  to  be  seen  only  in  the  cortical  cells  adjacent  to  the 
primary  canals  and  in  the  few  innermost  cells  of  the  cortical  rays.  The 
amount  is  so  small  here  that,  while  it  may  readily  be  seen  with  the  eye, 
the  photograph  does  not  differentiate  it. 

HYPOCOTYL. 
PRIMARY  STRUCTURE. 

The  primary  cortex  consists  of  six  layers  of  cells,  including  the 
endodermis.  The  epidermis  becomes  rather  strongly  cuticularized  and 
many  of  the  cells  are  papillate,  or,  more  correctly  speaking,  form  short, 
round-ended  trichomes,  which  are  usually  one-celled,  though  two-celled 
trichomes  are  found  in  a  few  instances  (plate  23 ,  fig.  9) .  The  angles  of  the 
cortical  cells  adjacent  to  the  inner  faces  of  the  epidermal  cells  are  collen- 
chymatized,  but  in  deeper  layers  this  character  is  not  present.  Chloro- 


Anatomy  and  Histology.  97 

plasts,  few  in  number,  however,  are  present  in  the  cortical  cells.  The 
endodermis  is  well  marked  and  contains  a  good  many  large  starch  grains. 
The  Casparian  spots  are  readily  recognized. 

The  stele  (0.18  mm.  in  diameter  in  a  hypocotyl  0.53  mm.  in  diameter) 
is,  at  an  early  age,  tetrarch  above  the  zone  of  transition  to  the  root.  The 
four  bundles  are  received  into  the  hypocotyl  in  pairs,  one  pair  from  each 
cotyledon,  in  which  they  constitute  the  median  trace.  After  reaching  the 
lower  part  of  the  lamina  they  unite,  as  they  do  in  the  lower  part  of  the 
hypocotyl  (in  the  "collet"),  to  form  a  single  bundle. 

In  addition  to  the  median  paired  cotyledonary  traces,  there  are 
delivered  into  the  hypocotyl  four  lateral  traces  which  meet  in  pairs  to 
constitute  two  bundles  which  pass  inward  and  downward.  Each  takes  a 
position,  the  one  on  one  side  of  the  stele,  the  other  on  the  other,  in  a  ver- 
tical plane  at  right  angles  to  that  which  divides  both  cotyledons.  They 
are  the  "  faisceaux  lateYaux  "  of  Dangeard  (1889,  p.  85).  So  far,  then,  this 
plant  satisfies  the  "cas  secondaire"  of  his  root  type  with  two  bundles, 
found  in  the  Compositae  and  certain  Ranunculaceae.  According  to  Dan- 
geard, however,  the  behavior  of  these  bundles  is,  to  use  his  own  words, 
as  follows:  "Les  premiers  (f.  median)  se  comportent  comme  dans  le  cas 
general;1  les  lateraux  s'anastomosent  plus  ou  moins  longuement  avant  de 
rejoindre  le  median  vers  le  bas." 

If  I  interpret  Dangeard's  statement  correctly,  we  should  find  that 
the  lateral  traces  (plate  24,  figs.  2  to  5,  12)  anastomose  with  the  median. 
This,  however,  I  do  not  believe  to  be  the  case.  By  following  the  figures  it 
will  be  seen  that  the  lateral  traces  are  to  be  seen  in  the  upper  part  of  the 
hypocotyl,  but  end  rather  soon.  The  broad  medullary  ray  between  the 
pairs  of  median  bundles  is  then  unoccupied,  and  remains  so  till  the  cauline 
bundles  encroach  upon  it.  Between  these  cauline  bundles,  at  the  proper 
level,  the  slender  end  of  the  lateral  cotyledonary  trace  may  be  seen,  quite 
single  and  separate  from  them  (plate  25^  fig.  10).  In  the  diagram  (plate 
24,  fig.  13)  the  fused  lateral  traces  are  represented  as  being  much  shorter 
than  in  that  given  by  Dangeard  for  Catananche  lutea. 

In  types  with  a  tetrarch  root-structure  this  trace  passes  downward 
and  articulates  directly  with  two  of  the  primary  hadrome  strands.  This, 
e.g.,  occurs  in  Caulophyllum  thalictroides  (Butters,  1909)  and  in  numerous 
other  plants  cited  by  Dangeard  (I.e.). 

The  intervals  between  the  lateral  and  median  bundles  are  occupied 
by  two  cauline  traces,  or,  more  properly  speaking,  by  one  (lateral  pro- 
phyllonary)  and  a  half  (of  the  corresponding  median  prophyllonary) 
traces.  There  thus  appear  in  the  higher  levels  of  the  hypocotyl: 

(1)  8  cotyledonary  traces,  viz,  2  pairs  of  half -median  traces;  2  pairs 
of  lateral  traces. 

(2)  8  prophyllonary  traces,  viz,  4  half-traces,  a  half-trace  on  each 
side  of  the  cotyledonary  laterals;  4  lateral  traces,  one  on  each  side  of  the 
cotyledonary  median  pairs. 

Passing  down,  the  i£  cauline  bundles  in  each  cotyledonary  median- 
lateral  interval  fuse  with  each  other  and  then  with  the  adjacent  median 
trace.  Below  the  level  of  this  fusion  the  tetrarch  structure  is  assumed, 

1  That  is,  as  above  described. 


98  Guayule. 

the  paired  cotyledonary  median  bundles  becoming  somewhat  separated. 
The  separation  is,  I  believe,  due  to  the  rapid  enlargement  of  the  adjacent 
parenchyma-cells,  so  that  the  secondary  elements  become,  in  the  lower 
part  of  the  hypocotyl,  definitely  dissociated,  leaving  the  primary  ele- 
ments occupying  the  position  of  the  primary  hadrome  elements  of  the 
root.  The  primary  hadrome  plate  of  the  root  lies,  then,  in  the  plane  of  the 
cotyledons  (Dangeard,  I.e.,  p.  87).  In  making  the  approach  to  the  root 
the  leptome  masses  revolve,  two  in  one  direction  and  two  in  the  other, 
until  they  meet,  two  and  two,  to  form  the  diametrically  opposed  leptome 
masses  of  the  root1  (plate  24,  fig.  2);  above,  these  same  leptome  masses 
pass  entirely  into  the  cotyledons,  with  the  corresponding  hadrome  masses, 
and  not  into  the  stem.  The  continuity  of  vascular  tissues  between  the 
stem  and  root  is  established  secondarily. 

The  above  account  of  the  structure  is  incomplete  in  that  the  presence 
of  an  originally  single  tracheal  vessel,  extending  from  within  the  cotyle- 
don downward  through  the  hypocotyl  into  the  root,  has  not  been  pointed 
out.  This  trachea  (trachee  primitive  of  Vuillemin,  1884,  P-  i&3)  consti- 
tutes a  center  of  development,  identical  in  the  hypocotyl  and  tap-root, 
for  the  primary  hadrome.  It  is  unnecessary  to  recount  the  arrangement 
of  hadrome  in  these  organs,  but  it  is  pertinent  to  insist  on  the  initial 
centripetal  formation  of  new  hadrome  elements.  The  dissociation  of  the 
hadrome  elements  in  the  hypocotyl — strictly  speaking,  only  in  the  upper 
portion — is  due,  as  Vuillemin  has  stated  (1884^,  p.  181),  to  the  rapid 
development  of  parenchyma,  and  is  analogous  to  the  secondary  splitting 
apart  of  the  wood  cylinder  in  the  same  organ  by  the  growth  of  the  con- 
junctiva. In  consequence  of  this  interpretation  Vuillemin  speaks  of  the 
paired  bundles  as  "les  deux  moities  du  faisceau,"  which  are  secondarily 
separated  by  "a  medullary  ray."  The  peculiar  orientation  of  the  paired 
bundles  represented  (but  frequently  not  referred  to)  by  many  observers 
(van  Tieghem,  Gerard,  Dangeard,  Goldsmith,  Ramaley)  is  thus,  properly 
I  believe,  explained.2 

PRIMARY  RESIN-CANALS. 

These  arise  in  the  endodermis,  as  in  the  root,  as  a  single  canal  directly 
opposite  each  primary  leptome  strand  (plate  25,  figs,  i  to  6).  The  struc- 
ture of  the  canal  is  similar  to  that  in  the  root,  and  consists  in  its  definitive 
form  of  eight  cells  in  transverse  section.  The  course  of  development  is 
not  so  regular  as  in  the  primary -root  canals,  the  meatus  being  ultimately 
more  cylindrical.  Not  infrequently  the  earlier  divisions  do  not  all  take 
place,  so  that  three  instead  of  four  cells  line  the  meatus  (plate  25,  fig.  2). 
As  these  canals  pass  into  the  root  they  pair  off,  each  pair  coming  to  occupy 
the  position  already  described,  namely,  opposite  each  of  the  two  primary 
leptome  bundles.  When  more  than  two  canals  are  encountered  in  this 
position  it  is  the  result  of  branching.  Occasionally  both  branch  either 
once,  or  frequently  twice,  giving  rise  at  length  to  four  or  even  six  canals, 
though  more  frequently  three  or  four  only  occur. 

1  The  above  account  may  be  applied  to  Parthenium  incanum,  Lactuca,  and 
Helianthus  in  its  main  outlines,  and  is  a  type,  I  believe,  of  wider  applicability  than 
usually  supposed. 

J  As  for  the  rest  of  Vuillemin's  views,  regarding  the  nature  of  the  hypocotyl 
and  cotyledons,  I  will  say  only  that  they  appear  to  me  somewhat  strained,  and 
far  less  in  accord  with  the  course  of  development  than  those  of  Dangeard  (1889). 


Anatomy  and  Histology.  99 

STEREOME. 

The  primary  stereome  arises  early  in  the  hypocotyl,  as  four  slender 
bundles  just  within  the  canals,  within  the  outermost  part  of  the  primary 
leptome  strands.  Occasionally,  also,  endodermal  cells  adjacent  to  the 
canal  may  undergo  sclerosis,  both  in  the  hypocotyl.  stem,  and  leaf  (plate 
25,  figs.  5,  6,  n). 

That  unmodified  pericycle  cells  lying  just  within  the  endodermis 
and  opposite  the  leptome  become  sclerosed  seems  possible  (plate  25,  fig.  6), 
but  doubtful.  I  find  that  the  pericycle  is  quite  frequently  interrupted 
(plate  22,  fig.  12),  in  which  event  the  stereome  must  arise  in  the  primary 
leptome.  Its  further  development  is  contributed  to  chiefly  by  elongated 
elements  in  the  leptome,  and  a  few  elements  are  sometimes  derived  also 
from  the  leptome  parenchyma.  Nearly  all  the  elements  (except  those  of 
parenchymatous  origin)  which  play  this  part  enlarge  greatly  (plate  25, 
fig.  4)  and  cause  marked  displacement  in  the  surrounding  tissues.  Vuille- 
min  (1884(1)  has  described  stereome  arising,  in  the  Compositae,  in  the  sec- 
ondary, but  not  in  the  primary  leptome,  in  Achillea,  Artemisia,  Anthemis, 
and  Leonto podium.  From  my  own  studies  I  am  forced  to  the  conclusion 
that  this  takes  place  in  the  primary  leptome. 

SECONDARY  STRUCTURE. 

The  prophyllonary  bundles,  above  referred  to,  arise  in  the  intervals 
between  the  cotyledonary  bundles,  before  the  establishment  of  inter- 
fascicular  cambium  (plate  24,  figs.  2  to  5,  12).  This,  when  complete, 
incloses  the  cotyledonary  hadrome,  and  there  is  thus  established  the  basis 
for  the  imposition,  on  the  primary  stele,  of  secondary,  true  stem  topog- 
raphy. It  may  be  pointed  out,  however,  that  the  cambium  does  not 
lay  down  secondary  hadrome  in  all  cases  in  immediate  contact  with  the 
primary  elements.  Thus,  in  the  radii  of  the  median  cotyledonary  traces 
(the  elements  of  which  do  not  of  course  pass  beyond  the  cotyledonary 
node)  secondary  hadrome  arises  which  descends  from  the  epicotyl  (plate 
25,  figs.  10  to  1 06).  Between  these  there  is  frequently  a  hiatus  which 
delimits  them  readily  to  the  eye,  if  the  secondary  changes  have  not  pro- 
ceeded too  far.  Nevertheless,  though  the  morphological  separateness  of 
the  primary  and  secondary  hadrome — and  also  leptome — is  clear,  the 
peculiar  topography,  the  curved  outline  of  the  secondary  hadrome  as 
seen  in  transverse  section,  indicates  an  as  yet  entirely  unanswered  ques- 
tion as  to  the  immediate  cause  of  this.  As  it  is  purposed  to  compare 
ecological  types,  further  detail  will  be  considered  in  the  following  para- 
graphs. 

FIELD  PLANTS. 

The  pith  in  a  specimen  about  1.8  mm.  in  diameter  displays  at  an 
appropriate  level  two  gaps  (plate  25,  fig.  7),  each  in  the  position  of  a 
primary  medullary  ray,  containing  the  primary  bundles1  constituting  the 
lateral  leaf-traces,  while  its  transverse  outline  still  reflects  the  vascular 
topography  of  the  primary  condition.  Surrounding  the  pith  is  a  closed 
compact  column  of  hadrome  which  is  broken  up  radially  into  broad  wedges 

1  These  undergo  little  or  no  secondary  thickening,  except  in  a  restricted  region 
below  the  cotyledonary  collar. 


100  Guayule. 

by  secondary  parenchyma  rays  (plate  25,  fig.  7).  The  primary  rays  are 
for  the  most  part  entirely  closed,  though  two  of  these  are  suggested  by 
the  topography  of  the  pith,  as  above  indicated.  No  more  resin-canals 
have  appeared.  The  primary  stereome  bundles  have  extended  inward 
by  the  transformation  of  the  primary  leptome,  and  the  primary  resin- 
canals  are  still  present.  Small  secondary  stereome  strands  are  present 
on  the  outside  of  several  other  bundles,  as  indicated  in  plate  25,  fig.  10. 
The  endodermis  is  recognizable  by  its  starch-content.  The  primary  cortex 
is  much  reduced,  its  tissue  having  been  sacrificed  to  the  development  of 
a  thick  cork,  the  original  peridermal  divisions  of  which  take  place  in  the 
outermost  cortical  layer  of  cells  (plate  23,  fig.  8). 

The  seedling  in  question  (plate  25,  fig.  7)  was  less  than  a  year  old, 
probably  four  to  six  months.  The  epicotyl  was  8  mm.  long,  with  a  few 
small  leaves,  and  was  collected  on  July  24,  1908. 

An  etiolated  seedling  (plate  25,  fig.  9)  of  the  same  diameter,  with  an 
epicotyl  2  cm.  long  and  about  three  months  old,  shows  a  similar  topog- 
raphy, save  quantitatively.  There  is  a  weaker  and  more  irregular  devel- 
opment both  of  hadrome  and  of  leptome.  There  is  no  additional  stereome 
beyond  the  four  primary  strands.  The  primary  cortex  is  thicker  and  the 
cork  thinner.  This  seedling  was  supplied  with  abundant  water  and  the 
shade  of  a  muslin  cloth,  with  the  effect  of  producing  responses  correlated 
with  a  relative  reduction  of  transpiration  and  to  loss  of  water  from  the  sur- 
face of  the  stem.  The  greater  leaf -area,  together  with  a  more  slender  axis, 
results,  however,  in  a  greater  transpiration  stream  relative  to  the  diameter 
of  the  wood  cylinder,  with  histological  results  to  be  noted  beyond. 

An  irrigated  plant  (plate  25,  fig.  8)  of  slow  growth,  one  which  was 
plentifully  supplied  with  water,  exposed  to  full  illumination,  but  limited 
in  the  spread  of  its  roots,  is  very  instructive  in  this  connection.  Under 
these  conditions  we  must  assume  a  strong  transpiration  stream,  at  least 
stronger  materially  than  is  usually  the  case  in  field  plants.  The  specimen 
had  a  diameter  of  2.5  mm.  and  was  not  more  than  three  months  old,  and 
on  this  account  alone  was  therefore  a  trifle  larger  and  more  advanced  in 
development  than  the  preceding.  In  its  cork  development  it  resembles 
the  field  plant,  and  has  suffered  the  same  reduction  of  the  primary  cortex. 
In  fact,  in  both  cases  one  of  the  primary  canals  is  just  cut  out  by  the  peri- 
derm.  The  deeper  medullary  rays  communicate  with  the  pith,  indicating 
secondary  enlargement  of  the  latter.  The  amount  of  wood  as  compared 
with  the  field  plant  is  much  greater  relative  to  age,  but  somewhat  less 
relative  to  radial  measurement,  and  there  is  a  relatively  larger  growth  of 
the  secondary  cortex.  Most  remarkable  is  the  large  and  irregular  devel- 
opment of  stereome.  This  irregularity  is  constantly  associated  with  a 
plentiful  water-supply  and  is  an  expression  of  a  general  tangential  dis- 
placement of  cortical  tissues,  as  revealed  by  the  later  positions  taken  by 
the  primary  resin-canals  and  the  obliquity  of  the  leptome  masses,  the 
position  of  which  predetermines  that  of  the  secondary  stereome. 

Aside  from  the  total  quantity  of  hadrome,  these  three  ecological 
types  present  histological  peculiarities  which  are  related  to  the  transpira- 
tion stream.  The  number  and  size  of  the  vessels  in  the  field  plant  (plate 
26,  fig.  2)  are  scarcely  inferior  to  those  in  the  irrigated  plants  (plate  26, 


Anatomy  and  Histology.  101 

fig.  4),  while  the  etiolated  plant  (plate  26,  fig.  3)  has  vessels  somewhat 
fewer,  but  of  more  uniform  size  and  notably  larger.  The  mechanical  ele- 
ments of  the  wood  are  thicker-walled  and  somewhat  smaller  in  the  field 
plant  (plate  26,  fig.  5),  and  are  nearly  isodiametric.  They  are  of  much 
the  same  character  in  the  other  two,  except  that  they  appear  more  com- 
pressed tangentially,  especially  in  the  irrigated  plant  (plate  26,  figs.  6,  7). 
The  stereome  also  presents  differences  which  are  still  more  striking, 
aside  from  the  relative  amounts  already  spoken  of.  In  the  field  (plate 
26,  fig.  8)  and  etiolated  (plate  26,  fig.  9)  plants  the  cells  are  closely  set 
together,  but  are  smaller  on  the  whole,  and  in  the  field  plant  have  smaller 
lumina.  In  the  irrigated  plant  (plate  26,  fig.  10)  the  shape  and  size  vary 
greatly,  the  lumina  are  very  small,  and  the  intercellular  material  is  much 
more  extensive.  The  whole  appearance  leads  to  the  impression  that  there 
is  a  good  deal  of  distortion  during  development,  so  that  the  fibers  are 
pushed  about  and  disarranged,  the  tissue  becoming  less  compact.  If  my 
view  of  the  origin  of  the  stereome  is  correct,  the  explanation  of  this  con- 
dition may  lie  in  a  less  complete  transformation  of  the  sieve-tissue  into 
stereome.  The  collapse  of  the  unsclerified  cells  would  cause  displace- 
ment, and  the  irregularities  due  to  change  in  position  and  unequal  growth 
of  the  stereomatic  cells  would  ensue.  The  more  slowly  growing  tissues 
are  the  more  regular  and  the  more  compact.  The  stronger  development 
of  mechanical  elements  in  irrigated  plants,  both  in  the  cortex  and  stele, 
appears  to  be  correlated  with  the  larger  growth  of  shoot,  while  the  larger 
vessels  of  the  etiolated  plant  indicate  the  greater  proportion  of  transpir- 
ing surface  (the  leaf-surface)  to  the  diameter  of  the  stem. 

LATER  SECONDARY  STRUCTURE. 

As  the  hypocotyl  approaches  a  diameter  of  3  mm.  a  total  movement 
outward  of  the  whole  vascular  system  (including  the  entire  wood  cylin- 
der) takes  place,  a  result  of  the  enlargement  of  the  pith  and  adjacent 
parenchyma-rays  tissue  (plate  26,  fig.  i ;  plate  28,  fig.  3).  The  inner  edges 
of  the  hadrome  plates  or  wedges  become  more  or  less  bent,  because  their 
edges  are  held  together  unequally  by  the  original  solid  mass  of  early  sec- 
ondary hadrome,  which  splits  usually  in  four  places,  corresponding  appar- 
ently with  the  primary  parenchyma  rays.  These,  therefore,  are  at  first 
closed  and  later  opened  secondarily,  as  shown  in  the  figure  (plate  25,  fig. 
8) ,  in  which  the  rupture  of  the  xylem  cylinder  is  beginning.  In  a  field 
plant  this  expansion  of  the  pith  is  also  accompanied  by  a  considerable 
tangential  growth  of  the  medullary  rays.  This  circumstance,  together 
with  the  relatively  slower  rate  of  growth  of  wood,  brings  about  the  result 
that  in  field  plants  (plate  25,  fig.  7)  the  amount  of  wood  is  relatively  less 
than  in  irrigated  plants  (plate  25,  fig.  8) ,  and  the  medullary  rays  are  wider. 
The  thickening  of  the  parenchyma  rays  is  shown  most  strikingly  in  an 
etiolated  seedling,  the  consequent  rupture  of  the  wood *  in  which  is  shown 
in  plate  26,  fig.  i. 

As  to  the  cortex,  the  growth  has  continued  in  all  of  its  parts  in  such 
a  manner  as  to  still  keep  the  primary  cortical  canals  included  within  the 

1  The  separation  of  the  young  hadrome  in  succulent  roots  in  this  manner  is 
well  known. 


102  Guayule. 

living  part.  Two  series  of  secondary  canals  have  arisen  in  the  hypocotyl 
of  the  size  under  consideration,  whether  the  growth  has  been  rapid  or 
slow  under  irrigation  (plate  29,  figs.  3,4),  or  slow  in  the  field  (plate  28, 
fig.  3) ;  but  the  total  number  of  canals  is  greater  in  the  irrigated  plants, 
as  would  be  expected  in  view  of  the  more  numerous  wood  plates.  The 
radial  depth  of  the  cork  has  not  increased  in  any  appreciable  amount  in 
the  field  plant,  but  is  more  uniform  than  in  a  rapidly  growing  plant,  in 
which  it  is  relatively  much  thinner  (plate  29,  fig.  3). 

It  is  of  interest  to  extend  our  comparison  to  rapidly  and  slowly  grow- 
ing irrigated  plants.  The  chief  point  of  difference  is  seen  in  the  much 
greater  tangential  development  of  sieve-tissue,  and,  later,  of  stereome, 
relatively  to  the  size  of  the  plant  in  slowly  growing  specimens  (plate  29, 
fig.  4).  This  statement  may  be  extended  also  to  the  mechanical  elements 
of  the  wood,  in  which  the  libriform  cells  are  of  smaller  diameter,  have 
smaller  lumina  and  are  more  cylindrical,  implying  a  greater  amount  of 
intercellular  cementing  substances.  The  vessels  too  are  of  smaller  diame- 
ter, and,  though  this  is  compensated  for  by  their  greater  numbers,  the 
capacity  of  the  vessels  in  the  more  rapidly  grown  plant  is  considerably 
greater  (plate  27,  figs.  6,  7).  The  phloem  presents  analogous  differences, 
having  in  the  slowly  growing  plant  a  structure  denser  and  much  more 
extended  tangentially  than  in  the  rapidly  grown  plant,  and  in  this,  as  in 
the  character  of  the  wood,  resembling  more  closely  the  field  plant  (plate 
28,  fig.  3).  A  still  further  difference,  of  more  fundamental  character 
morphologically,  is  the  development,  in  slowly  growing  irrigated  plants, 
of  stereids  in  the  pith  (plate  29,  fig.  4).  So  far  as  I  have  been  able  to 
observe,  the  stereids  occur  under  no  other  condition  in  the  hypocotyl, 
though,  as  will  be  shown,  it  occurs  normally  in  the  pith  in  the  definitive 
stem  (plate  29,  figs.  5,  6). 

The  observations  on  the  structure  of  the  wood  in  the  seedlings  studied, 
regarding  especially  the  water-carrying  elements,  are  of  peculiar  interest 
as  they  stand  in  relation  to  those  of  Cannon  (1905),  who  studied  compar- 
atively irrigated  and  non-irrigated  desert  woody  plants  of  eight  species. 
His  general  conclusions,  undoubtedly  supported  by  his  observations,  are 
that  "there  can  be  no  mistaking  the  fact  that  branches  of  irrigated  plants 
(even  if  semi-irrigated  only)  are  poorer  in  conductive  tissue  than  branches 
of  the  same  diameter  of  non-irrigated  plants,"  but  he  says  at  the  same 
time  that  this  "is  an  unexpected  condition."  Further,  "irrigated  plants 
organize  each  year  a  larger  amount  of  wood — which  contains  a  relatively 
large  amount  of  non-conductive  tissue — so  that  it  happens  that  non-irri- 
gated and  older  stems  have  more  vessels  than  irrigated  and  younger"  of 
the  same  diameter. 

For  the  reason  that  I  found,  to  my  surprise  also,  that  some  of  my 
observations  coincide  with  Cannon's,  I  venture  to  cite  certain  concrete 
instances,  and  state  these,  together  with  those  already  presented,  in  brief 
fashion,  by  way  of  instituting  a  comparison  of  our  results: 

i .  In  field  plants  (the  seedlings  above  studied)  the  vessels  are  as  large 
as  in  irrigated  plants  of  slow  growth,  or  larger,  and  are  slightly  more 
numerous.  The  stems  are  of  nearly  equal  diameter  (plate  26,  figs.  2,4; 
plate  2 7,  figs.  6, 8). 


Anatomy  and  Histology.  103 

2.  On  the  contrary,  in  irrigated  seedlings  of  very  rapid  growth  the 
vessels  are  much  larger,  though  not  quite  so  numerous,  as  in  the  plants 
mentioned  under  (i) ;  but  the  total  amount  of  wood  is  considerably  greater 
relative  to  the  diameter  of  the  stem  (plate  27,  fig.  7). 

3.  The  terminal  twig  of  a  field  plant  of  very  large  size,  in  which  the 
amount  of  growth  in  any  twig  was  very  small  in  one  season,  is  contrasted 
with  an  irrigated  twig  of  rapid  growth.    The  wood  cylinders  are  equal  in 
diameter ;  the  vessels  are  somewhat  larger  in  the  secondary  xylem  of  the 
field  plant.     But  in  the  protohadrome  the  vessels  are  larger  in  the  irri- 
gated plant  (plate  27,  figs.  4,  5).    Both  twigs  of  the  same  and  last  season's 
growth. 

4.  Two  twigs  of  about  the  same  diameter  of  wood  cylinder,  one  a  field 
twig  two  years  old,  the  other  irrigated,  one  year  old.    The  total  number 
of  vessels  is  greater  in  the  field  plant,  and  there  are  more  large  and  more 
smaller  vessels.     In  the  protohadrome,  however,  the  reverse  as  regards 
size  is  true.     But  the  number  of  vessels  in  either  year's  hadrome  in  the 
field  plant  is  probably  the  same  as,  or  is  less  than,  that  in  the  irrigated 
plant  (plate  27,  figs.  9,  10). 

5.  On  the  contrary,  in  another  irrigated  stem  6.5  mm.  in  diameter, 
the  number  and  size  of  the  vessels  are  enormously  superior  to  the  number 
and  size  in  a  field  plant  (plate  27,  figs.  2,3). 

6.  The  protohadrome  in  a  field  seedling  of  usual  growth  compared 
with  that  of  an  irrigated  plant,  before  secondary  xylem  has  appeared  in 
either  case.    In  the  irrigated  plant,  in  which  growth  is  rapid,  the  elements 
in  question  are  much  larger  (plate  26,  figs,  n,  13). 

7.  The  protohadrome  in  a  peduncle,  through  which  there  is,  relative 
to  its  size,  it  can  hardly  be  doubted,  a  very  large  transpiration  stream,  is 
composed  of  very  large  elements  (plate  26,  fig.  12). 

8.  In  an  etiolated  seedling  (plate  26,  fig.  3),  in  which  the  size  of  the 
stem  remains  small  in  relation  to  the  total  transpiring  area,  the  size  of  the 
conducting  elements  is  greater,  and  their  numbers  scarcely  less,  than  in  a 
field  or  irrigated  seedling  of  approximately  the  same  size  of  stem. 

9.  In  the  tap-root  of  very  rapidly  grown  seedlings  the  vessels  are 
much  larger  and  the  amount  of  mechanical  tissue  much  less. 

These  observations  are  in  part  antagonistic,  in  appearance  at  any 
rate,  to  those  of  Cannon,  and  in  part  agree  with  them.  They  must  there- 
fore be  harmonized  among  themselves  as  well  as  with  Cannon's.  In  at- 
tempting to  cover  all  the  cases  with  one  explanation,  we  must  not  forget 
that  the  problem  indicated  is  a  complex  one,  inasmuch  as  the  ratios  of 
mechanical  tissues  in  the  two  types  enter  into  it.  It  will,  however,  suffice 
to  speak  of  the  conducting  elements  alone  at  the  present  moment. 

In  stems  of  guayule  of  a  given  diameter  in  field  and  irrigated  plants, 
the  amount  of  wood  is  greater  in  the  latter.  In  wood  cylinders  *  of  equal 
diameter  the  same  holds  true.  This  is  due  to  (a)  the  smaller  amount  of 
cortex  in  irrigated  plants  and  (6)  the  narrower  medullary  rays.  We  may 
assume  that  the  growth  in  thickness  of  the  stem  is  correlated  with  the 
growth  of  the  shoot  above.  In  the  same  period,  the  total  amount  of  con- 

1  Wood,  medullary  rays,  and  pith  taken  as  a  whole. 


104  Guayule. 

ducting  tissue  formed  in  irrigated  wood  is  undoubtedly  much  greater  than 
in  that  of  field  plants,  but  the  amount  of  mechanical  tissue  is  also  greater. 
Putting  these  facts  together,  it  seems  reasonable  to  conclude  that  the 
•capacity  of  the  conducting  elements  is  correlated  with  the  maximum  transpi- 
ration stream.  The  relative  numbers,  and  therefore  their  size,  depend 
primarily  upon  other  conditions  productive  of  the  development  of  me- 
chanical elements.  On  comparing  the  shoots  of  field  and  irrigated  plants, 
it  is  clear  that  the  mechanical  conditions  in  the  latter  are  those  under 
which  mechanical  tissue  would  be  developed.  The  mere  weight  of  the 
foliage  alone  would  be  expected  to  insure  such  responses. 

ADVANCED  SECONDARY  CONDITION  OF  THE  HYPOCOTYL. 
In  a  more  advanced  stage  of  growth  nothing  of  especial  note,  be- 
yond that  already  pointed  out,  presents  itself  for  discussion.  One  point, 
however,  is  worth  noting,  namely,  that  the  daughter  and  granddaughter 
cells  of  the  cortex  remain  arranged  in  tetrads  chiefly,  thus  giving  the 
whole  tissue  the  appearance  of  consisting  of  pairs  and  tetrads  of  cells. 
The  original,  but  enlarged,  intercellular  spaces  are  very  much  in  evidence 
(plate  28,  fig.  4) .  Regularly  shaped  and  disposed  spaces,  such  as  have  been 
described  for  the  root,  do  not  occur  in  the  stem. 

AGE  AND  STRUCTURE  IN  THE  SEEDLING. 

Both  popular  and  scientific  discussion  frequently  turn  on  the  corre- 
lation of  age  and  structure  in  the  guayule.  Inasmuch  as  the  hypocotyl  is 
the  oldest  portion  of  the  stem,  it  is  worth  while  to  indicate  the  structure 
of  field  plants  of  known  age.  A  seedling  from  Station  2,  which  was  less 
than  one  year  old  when  collected  in  April  1909,  with  a  stem  5  cm.  long 
and  4.6  mm.  in  diameter  at  the  base,  shows  in  the  hypocotyl  the  struc- 
ture represented  in  plate  30,  fig.  i.  The  living  cortex  (primary)  is  very 
sharply  delimited  from  the  cork  on  account  of  the  rubber-content  of  the 
living  cells.  It  will  be  seen  that  the  specimen  closely  resembles  the  slowly 
grown  irrigated  plant  above  described,  while  in  point  of  fact  it  is  a  plant 
of  rapid  growth  for  field  conditions,  being  much  above  the  average  size  for 
the  locality  in  which  it  was  collected.  It  is  seen  from  this,  what  will  in 
any  event  be  understood,  that  all  field  plants  are  not  alike,  the  water- 
supply  varying  at  different  times  in  different  habitats,  thus  inducing  at 
times  growth  quite  similar  to  that  which  usually  occurs  under  more 
favorable  conditions.  This  seedling  has,  in  addition  to  the  four  primary 
canals,  three  series  of  secondary  canals.  One  below  the  average  size,  of 
the  same  age,  with  an  epicotyl  8  mm.  long,  2.4  mm.  in  diameter,  has  only 
the  four  primary  canals.  These  are  finally  thrown  out  when  a  diameter 
of  6  to  7  mm.  is  attained,  and  therewith  the  whole  of  the  primary  cortex 
is  lost. 

In  a  seedling  of  the  same  diameter  three  years  old,  it  is  possible  to  see 
three  annual  rings  of  wood,  marked  by  the  larger  pores  of  the  first  growth 
of  each  season.  There  are  in  the  same  stem,  aside  from  the  four  primary 
canals,  three  series  of  resin-canals,  one  in  the  primary  and  two  in  the 
secondary  cortex,  so  that  there  are  marks  of  three  zones  of  cortex,  the 
primary  and  two  secondary,  corresponding  apparently  with  the  three 


Anatomy  and  Histology.  105 

seasons  of  growth.  Comparing  the  two  cases,  we  find  that  the  structure 
attained  in  the  cortex  by  a  seedling  of  one  season  may  be  the  same  as  that 
attained  in  three  years  by  one  of  slower  growth,  while  the  number  of 
growth-periods  is  reflected ,  albeit  frequently  only  indistinctly ,  by  the  wood . 
It  is,  however,  generally  true  that  the  ring-structure  may  be  made  out. 

EP1COTYL. 

Seedlings  partially  etiolated  by  being  grown  under  a  muslin  screen, 
in  which  the  internodes  have  lengthened,  render  the  analysis  of  the  tis- 
sues easier.  The  lowermost  internodes  of  such  seedlings  receive  primarily 
six  bundles  (plate  24,  fig.  5)  from  the  hypocotyl,  but  the  number  is  at 
once  increased,  so  that  immediately  above  the  base  eight  or  even  more 
bundles  may  be  counted  (plate  24,  fig.  6).  The  increase  is  more  marked 
in  plants  with  short  internodes,  and  the  primary  condition  is  quickly 
masked.  The  development  of  the  stereome  which  arises  in  the  primary 
leptome  is  in  the  primary  numerical  relation,  there  being  at  first  six 
bundles,  opposite  the  median  and  lateral  leaf-traces  of  the  first  two  foliage 
leaves.  These  relations  are  shown  very  beautifully  in  a  section  taken 
from  a  seedling  which  had  developed  one-sidedly,  and  this  is  figured  in 
plate  30,  fig.  2.  The  relations  of  the  primary  cortical  canals  received  from 
the  hypocotyl  are  well  shown  also  in  this  section.  One  pair  of  these 
accompanies  the  median  leaf-trace  of  the  first  leaf,  the  other  pair  that  of 
the  second  leaf.  The  third  petiole  may  receive  two  or  one,  and  this  is  true 
of  all  the  earlier  leaves  as  far  as  the  tenth  node,  approximately.  The 
primary  condition,  that  in  which  two  lateral  canals  occur,  may  recur  even 
in  later  stages  of  development,  but  only  infrequently.  As  they  pass  into 
the  leaf  one  becomes  smaller  and  ends  blindly  (plate  38,  figs.  3  to  9), 
while  the  other  extends  into  the  leaf-blade.  In  this  there  is  a  striking 
similarity  between  the  earlier  foliage  leaves  and  the  cotyledons,  constitut- 
ing a  morphological  argument  against  the  theory  that  the  cotyledons  are 
not  primitively  leaves.  The  absence  of  medullary1  stereome,  mentioned 
above  in  the  paragraphs  dealing  with  the  hypocotyl,  will  be  noted,  and 
this  condition,  as  in  the  case  of  retonos,  persists  until  the  level  of  the 
tenth  internode  or  thereabout.  Sections  of  field  seedlings  with  short  inter- 
nodes at  a  distance  of  several  millimeters  from  the  insertion  of  the  coty- 
ledons show  no  medullary  stereome,  and  this  is  true  also  of  medullary 
canals. 

The  same  section  (plate  30,  figs.  3,  4)  serves,  in  addition,  to  show 
very  clearly  the  origin  of  the  periderm,  which  in  the  definitive  stem,  as  in 
the  earlier  internodes,  occurs  in  the  first  or  outermost  cortical  layer  of  cells 
(as  shown  by  Ross,  1908).  Fron  and  Frangois  (1901)  state  differently, 
and  their  drawing  depicts  the  earliest  suberogenous  divisions  in  the 
second  layer  of  cells;  in  this,  however,  they  are  in  error.  Their  drawing  is 
taken  from  a  section  through  the  base  of  a  petiole,  as  the  position  of  the 
leaf-traces,  so  labeled,  shows.  In  such  a  section,  it  is  true,  the  earlier 
divisions  will  be  seen  in  the  second,  third,  or  even  fourth  layer.  I  have 
introduced  two  figures  taken  from  portions  of  the  tissue  in  question  on 

1 1  use  this  in  a  purely  descriptive  sense.  " Perimedullary  stereome"  has  been 
used.  The  origin  of  this  stereome  is  dealt  with  beyond  (p.  no). 


106  Guayule. 

opposite  sides  of  the  same  section.  In  the  position  opposite  the  first  leaf- 
trace  the  divisions  occur  in  the  second  layer;  at  the  other  end  of  the 
diameter,  in  the  outermost.  The  periderm  figured  by  Fron  and  Francois 
is  therefore  the  leaf  absciss  layer.  Leaf  fall  in  the  guayule  is  consum- 
mated only  slowly,  and,  as  compared  with  more  familiar  examples  of  the 
temperate  regions,  is  imperfect  in  its  time  relations.  The  layer  is  not 
sharply  defined,  and  the  disintegration  of  the  tissue  is  irregular,  the  result 
of  the  uneven  and  irregular  character  of  the  component  cells  of  the 
absciss  layer. 

The  epidermis,  both  of  stem  and  leaves,  in  the  epicotyl  is  clothed 
with  a  single  type  of  trichome  found  throughout  (plate  30,  figs.  5  to  n). 
There  are  two  derived  kinds,  a  T-shaped  hair  predominating,  with  a  few 
scattered  hairs  of  a  type  seen  in  Chrysoma  (Lloyd,  1901)  and  in  other 
Compositae,  viz,  the  whip-hair,  but  in  which,  in  the  guayule,  the  terminal 
cell  remains  undeveloped.  The  trichome  does  not,  therefore,  become 
flagellate,  as  in  the  related  species,  the  mariola  (Parthenium  incanum] ,  and 
in  many  Compositas  (Vesque,  1885).  In  certain  places,  as  in  the  axils  of 
the  leaves,  floral  bracts,  and  corolla,  transition  forms  may  be  met  with, 
indicating  that  the  two  kinds  have  been  derived  phylogenetically  from  a 
single  type.  The  fact  that  both  are  present  in  different  Compositae,  but 
in  different  ratios,  may  be  used  to  support  the  view  that  the  trichome 
clothing  is  a  character  which  has  been  brought  about  by  gradual  change 
and  not  by  the  sudden  dropping  out  of  one  or  the  other  kind.  The 
T-shaped  hairs  clothe  the  plant  very  completely  and  smoothly,  the  termi- 
nal cells  all  lying  very  nearly  parallel  to  each  other,  and  to  the  axis,  on 
the  various  organs.  The  density  of,  the  covering  varies,  however,  with  the 
size  of  the  organ,  as  the  individual  hairs  show  no  substantial  amount  of 
response  to  varying  external  conditions.1 

Before  leaving  this  part  of  the  subject  it  is  necessary  to  point  out 
that  in  seedlings  in  which  the  stem  elongates  slowly,  as  in  the  field,  the 
primary  cortical  canals  of  the  hypocotyl  behave  in  a  manner  which  has 
not  been  observed  in  etiolated  plants.  The  two  pairs,  associated  with  the 
median  leaf -traces  of  the  two  early  foliage  leaves,  instead  of  passing  directly 
into  the  petioles,  anastomose  and  then,  from  the  transverse  lacuna  thus 
formed,  canals  pass  off  to  enter  the  leaves.  Other  canals  have  been  noted 
to  rise  from  the  lacuna  and  to  pass  up  into  the  epicotyl;  a  reanastomosis 
within  a  short  distance  has  been  observed  (plate  36,  figs.  7,8).  A  section 
of  a  field  seedling  made  through  the  cotyledonary  node,  or  at  any  level, 
if  the  internodes  are  undeveloped,  will  almost  invariably  show  widely 
spreading  divarication  of  one  or  more  of  the  canals  (plate  36,  fig.  6).  In 
a  word,  the  canals  constitute  a  branching  system,  each  more  or  less  in 
communication  with  the  other. 

1  The  mechanical  conditions  in  axils  of  leaves  and  in  the  capitula  cause  super- 
ficial changes  in  the  shapes  of  the  trichomes. 


Anatomy  and  Histology.  107 

THE   DEFINITIVE   STEM. 
PRIMARY  STRUCTURE. 

After  the  tenth  internode,  approximately,  has  been  laid  down,  the 
stem  takes  on  its  definitive  structure.  The  number  and  appearance  of  the 
various  structures  within  the  growing  tip  vary  a  good  deal,  according  to 
the  rate  of  growth.  This  is  largely  due  to  the  crowding  together  of  the 
nodal  characters,  but  in  part  also  to  the  size  of  the  terminal  bud,  and 
therefore  to  the  number  of  leaves.  In  a  thick  apex  more  bundles  of 
primary  elements  appear  at  a  given  level  (plate  31,  fig.  3) ;  also  the  size 
and  frequency  of  branching  of  the  canals  is  greater  within  a  given  zone 
(plate  36,  fig.  5).  For  the  purpose  of  description  it  will  serve  to  present 
briefly  the  differences  observed,  at  various  levels  of  a  stem  one  year  old,  of 
normal  growth-rate  in  the  field,  as  this  will  give  an  epitome  of  the  develop- 
ment of  the  tissues.  The  specimen  before  me  is  a  twig  which  grew  in  1 908, 
collected  at  the  close  of  its  elongation  for  that  season.  It  is  1 1  cm.  long, 
4  mm.  in  diameter  at  the  base,  and  1.6  mm.  just  behind  the  tip.  The 
structure  at  the  levels  mentioned  is  as  follows: 

Within  the  last  millimeter  of  the  tip  one  finds  the  vascular  tissues 
undifferentiated,  though  the  medulla  and  vascular  zone  are  recognizable. 
The  primary  cortical  canals  appear  opposite  median  leaf-traces,1  but 
nowhere  else  (plate  38,  fig.  i ;  plate  31,  fig.  5).  The  starch  sheath  (endo- 
dermis)  is  recognizable  only  by  the  starch-content,  which  appears  only 
opposite  leaf-traces,  while  starch  is  absent  from  the  endodermis  else- 
where. Within  half  a  millimeter  further  down,  at  a  diameter  of  a  milli- 
meter, all  the  5  medullary  canals  appear,  17  vascular  bundles  are  distinct, 
and  1 6  cortical  canals  are  present  (plate  31 ,  fig.  4).  In  perhaps  half  of  the 
bundles  spiral  vessels  have  developed.  These  are  in  curved  plates  of  i  to  3 
vessels,  each  separated  by  wood-parenchyma.  The  epidermis  is  densely 
clothed  with  T-shaped  trichomes.  The  endodermis  may  be  traced  com- 
pletely around  the  stele,  on  account  of  its  starch.  At  this  level  may  be  seen 
the  earliest  indications  of  the  stereome  bundles  in  the  primary  leptome  and 
in  the  pith. 

At  10  mm.  from  the  apex  (diameter  2.5  mm.)  the  collenchyma  of  4  to 
6  rows  is  well  developed.  The  characteristic  thickening  is  first  seen  in  the 
periclinal  walls,  and  these  become  still  more  conspicuously  thickened 
in  the  later  stages.  The  larger  bundles  have  xylem  plates  6  to  8  cells 
deep  radially.  Interfascicular  cambium  is  being  developed.  The  stere- 
ome is  still  thin- walled,  but  the  definitive  size  of  the  cells  has  been  reached, 
and  thickening  has  taken  place  at  the  angles.  In  the  section  before  me  I 
count  2  5  primary  cortical  canals  and  i  o  medullary  canals.  The  section  was 
evidently  taken  just  above  the  plane  in  which  the  pith-canals  branched,  as 
two  of  the  canals  are  cut  at  the  fork. 

At  15  mm.  (diameter  3  mm.)  mechanical  elements  have  appeared  in 
the  hadrome,  and  the  stereome  is  more  advanced  as  to  the  thickening  of 
the  walls.  The  collenchyma  has  been  somewhat  stretched  periclinally, 
the  walls  so  placed  being  much  thicker.  The  walls  of  both  cortical  and 

1  Very  occasionally  a  pair,  a  single  one  on  each  side  of  the  trace,  occurs. 


108  Guayule. 

pith  cells  have  thickened,  and  in  the  walls  of  contact  the  reticulations,  due 
to  the  broad,  ovate,  closely-set  pits,  are  very  noticeable.  The  interspaces 
are  large. 

At  various  lower  levels,  depending  on  the  time  of  the  year  in  which 
the  material  is  taken,  will  be  encountered  the  young  periderm.  Ross  * 
speaks  of  this  as  beginning  very  early,  and  in  his  material  as  reaching  close 
to  the  apex.  If  a  newly  grown  twig  is  examined  toward  the  close  of  the 
season  it  will  be  found  that  the  periderm  embraces  only  a  lower  zone  (of  a 
thickness  depending  on  the  rate  of  growth)  at  the  base  of  the  stem,  and  its 
growth  involves  casting  off  the  leaves  which  remained  on  the  upper  por- 
tion of  the  twig  of  the  previous  year.  This  uppermost  zone,  carrying  the 
overwintered  leaves,  undergoes  some  growth  with  considerable  lengthening 
of  the  internodes,  so  that  the  leaf -scars  of  the  winter  bud  do  not  crowd 
each  other  as  do  the  bud-scale  scars  in  plants  of  the  temperate  regions. 
The  periderm  passes  upward  from  this  zone,  and  during  the  following  dry 
season  slowly  cuts  away  the  leaves,  until  by  midwinter,  earlier  or  later 
according  to  the  character  of  the  season,  all  the  leaves  of  the  previous 
growing  season,  save  the  terminal  ones,  are  cast  off  (plate  14,  fig.  B). 
As  the  periderm  extends  toward  the  apex  of  the  twig  the  epidermis  is 
fissured  concurrently,  beginning  at  the  base. 

A  section  near  the  base  of  the  season's  growth  shows  the  following 
structure:  The  periderm  is  three  to  four  cells  deep,  measuring  o.i  mm. 
The  xylem  bundles  measure  about  0.5  mm.  on  the  radius,  and  the  pith  has 
a  diameter  of  i  mm.  Nearly  all  the  bundles  are  supplied  with  both  corti- 
cal and  medullary  stereome.  Tracheids  are  fewer  in  the  outermost  zones 
of  the  xylem.  The  primary  cortical  canals  and  pith  canals  have  generally 
enlarged,  the  largest  measuring  0.3  to  0.4  mm.  tangentially,  with  a  radial 
diameter  of  0.15  to  0.2  mm.  This  section  has  one  completed  series  of  sec- 
ondary cortical  canals,  and  a  second  row  begun.  The  epidermis  is  slightly 
fissured.  This  amount  of  growth  and  secondary  change  is  by  no  means  the 
maximum  possible.  The  thickest  part  of  the  stem  of  one  season's  growth 
of  the  seedling  shown  in  plate  46,  fig.  A,  had  five  series  of  secondary 
canals,  and  cork  0.5  mm.  thick,  the  depth  of  the  cortical  tissues,  primary 
and  secondary,  being  2.5  mm. 

A  stem  of  two  growth-periods  shows  the  primary  and  one  series  of 
secondary  canals,  but  the  two  seasons'  accretions  of  wood  are  reflected  in 
the  annular  structure  of  the  wood,  as  in  the  seedling  hypocotyl  before 
mentioned.  Here  also,  therefore,  the  relation  of  structure  to  age  is  less 
apparent  in  the  cortex  than  in  the  wood  cylinder.  The  whole  of  the  outer 
leptome  (that  embraced  between  the  primary  and  secondary  series  of 
canals),  is  stereomatic;  that  within  the  secondary  series  still  retains  its 
sieve  character.  A  considerable  thickness  of  cork  has  developed. 

Later  changes  need  not  be  followed  year  by  year,  and  it  will  suffice 
to  point  out  the  more  important  features  summarily.  The  inner  periderm 
normally  does  not  begin  until  the  stem  attains  a  diameter  of  over  10  mm. 
(Ross,  /.  c.),  and  the  primary  cortical  canals  may  still  be  found  up  to  this 
time  or  even  very  much  later,  e.g.,  in  a  stem  28  mm.  in  diameter,  with  cor- 
tex, including  bark,  5  mm.  thick.  The  penetration  of  the  inner  periderm 

1  His  material  appears  to  have  been  collected  in  December. 


Anatomy  and  Histology.  109 

is  not  a  clean-cut  process,  such  as  we  see  in  our  common  trees  and  shrubs, 
but  first  appears  directly  opposite  either  a  primary  stereome  bundle  or  a 
primary  canal,  as  an  ingrowth,  simulating  invagination  (plate  32,  fig.  4). 
The  absciss  layer  which  effects  leaf -fall  is  similarly  clumsy,  so  to  speak. 
This  tissue  consists  of  a  quite  irregular  layer  of  cork-cells,  continuous  with 
the  primary  phellogen.  The  outermost  cells,  those,  namely,  immediately 
beneath  the  base  of  the  leaf,  first  become  suberized. 

Until  an  advanced  age  is  attained,  the  inner  periderm  does  not  cut 
deeply.  In  old  stems,  20  to  50  years  of  age,  light-colored  layers  of  cork 
may  be  seen  penetrating  to  half  the  depth  of  the  cortical  tissues,  but  quite 
irregularly.  It  is  of  interest  to  note  here  that  this  cork  presents  a  special 
practical  difficulty  in  the  factory  in  handling  the  comminuted  shrub  after 
it  has  passed  through  the  pebble-mill.  The  bagasse  is  then,  with  the 
exception  of  this  cork,  which  has  been  broken  up  into  flakes,  separated  in 
water,  the  rubber  and  the  cork  flakes  floating  and  the  remainder  sinking. 
Only  by  means  of  pressure  under  water  or  prolonged  soaking  may  the 
cork  be  waterlogged,  when  it  sinks,  leaving  the  clean  rubber  still  floating. 
These  layers  of  cork  are  seen  in  plate  2,  fig.  B,  from  a  photograph  of  a 
stem  certainly  forty  years  old. 

SECONDARY  STRUCTURE. 

The  secondary  cortex  is  characterized  by  alternating  concentric  rows 
of  stereome  bundles  and  resin-canals.  Between  succeeding  stereome  mass 
and  leptome  parenchyma  (canal-cells,  consisting  of  endothelium  and  usu- 
ally a  single  subjacent  or  supporting  layer) ,  there  frequently  intervenes 
no  tissue  at  all,  and  the  stereome  occupies  the  whole  of  the  space  between 
adjacent  resin-canals.  In  the  inner  part  of  the  secondary  cortex  one  finds 
alternating  canals  and  "soft"  leptome,  the  composition  of  which  raises 
some  points  of  question.  The  canals,  as  described  correctly  by  Ross 
(1908),  arise  as  a  double  row  of  cells  derived  directly  from  the  cambium 
(plate  22,  fig.  13).  Surrounding  the  "  secreting  "  cells  is  at  least  one  layer 
of  leptome  parenchyma,  the  usual  condition  in  slowly  growing  plants.  In 
irrigated  plants  there  may  be  two  or  three  (or  occasionally  more)  layers 
(plate  29,  fig.  i).  This  is  followed,  radially,  by  a  mass  of  sieve-tissue 
(plate  32,  fig.  2),  which  may  be  regular  in  transverse  outline,  and  com- 
pletely uninterrupted  by  parenchyma  until  another  canal  is  laid  down, 
or  it  may  be  narrow  and  more  or  less  irregular,  as  in  irrigated  plants 
(plate  25,  fig.  8;  plate  29,  fig.  4).  In  any  event,  the  sieve-tissue  occupies 
the  radially  placed  space,  broadly  speaking,  between  successive  canals, 
and  it  is  in  this  space  that  we  find  stereome  later. 

The  manner  in  which  the  stereome  arises  is,  in  broad  outline,  as 
follows:  The  outermost  (on  the  radius)  leptome  cells  undergo  transverse 
enlargement  and  become  stereo matic.  Successively  other  adjacent  cells 
lying  farther  in  behave  similarly.  The  resulting  tissue,  however,  occu- 
pies more  space  than  did  the  original  cells  from  which  it  arose.  As  the 
total  space  which  is  occupied  by  the  stereome  is  usually  identical  with  the 
total  leptome,  it  follows  that  there  must  be  some  readjustment.  This  is 
brought  about  by  the  discontinuous  sclerosis  of  the  leptome,  so  that  irreg- 
ularly alternating  masses  of  this  are  destroyed  and  become  compressed. 


110  Guayule. 

The  stereome  develops,  therefore,  within  the  leptome,1  and  in  its  defin- 
itive form  a  portion  of  the  leptome  comes  to  occupy  the  volume  of  the 
whole.  The  definitive  stereome  may  be  flanked  more  or  less  completely 
by  sclerosed  leptome  parenchyma,  and  even  the  adjacent  cortical  cells, 
especially  in  the  peduncle,  may  take  on  this  character. 

With  reference  to  origin,  in  general  terms,  I  am  at  variance  with 
Ross,  who  says  on  this  point:  "Durch  die  Tatigkeit  des  Kambiums  ent- 
stehen  abwechselnde  Gruppen  von  zartwandigen  Elementen  und  von  Scle- 
renchymfasern.  In  der  jiingsten  Gruppe  der  letzteren  geht  die  Verdick- 
ung  der  Zellwande  erst  sehr  allmalich  vor  sich,  und  in  den  zartwandigen 
Schichten  zwischen  dieser  und  dem  Kambium  kommt  der  Secret  kanal 
zur  Ausbildung."  I  believe  that  I  am  not  unduly  criticizing  Ross's  state- 
ment by  saying  that  it  is  misleading.  It  would  seem  more  consonant  with 
the  facts  to  say  that  through  the  activity  of  the  cambium  alternating 
groups  of  leptome  parenchyma  and  prosenchyma  arise,  and  that  the 
stereome  arises  within  the  latter.  The  resin-canals  arise  from  two  adja- 
cent tangential  layers  of  the  thin- walled  parenchyma. 

The  change  of  any  particular  cells  into  stereome  is  not  complete 
before  the  end  of  the  third  season's  growth,  as  nearly  as  we  may  judge. 
This  secondary  occupation  of  the  leptome  by  the  stereome  is  particuarly 
well  shown  in  a  preparation  made  of  the  cortex  of  an  old  stem  (plate  32, 
fig.  2).  The  sections  were  treated  with  xylol  so  as  to  extract  the  rubber, 
leaving  the  tissues  empty  and  distinct.  The  stereome  was  seen  to  occupy, 
with  few  exceptions,  all  the  space  previously  occupied  by  the  sieve-tissue. 

ORIGIN  OF  THE  MEDULLARY  AND  CORTICAL  STEREOME. 
Vuillemin,  1884^,  p.  223,  has  described  stereome  as  arising  in  the 
pericycle  in  the  Composite,  but  he  does  not  show  its  precise  origin  nor 
that  of  its  constituent  elements ;  nor  does  his  description  of  the  leptome 
(I.e.,  p.  99)  fit  the  conditions  found  in  Parthenium  argentatum.  Accord- 
ing to  Vuillemin,  the  sieve-tubes  are  of  much  larger  transverse  diameter 
than  the  companion  cells,  and  this  is  not  true  of  our  plant.  There  are, 
however,  broad  elements  with  oblique  end-walls,2  intermixed  with  sieve- 
tubes  and  companion-cells  to  form  a  melange  in  which  the  sieve  elements 
are  generally  in  contact  with  each  other  throughout  the  whole  leptome 
mass,  and  do  not  usually  form  isolated  islands,  as  generally  described  for 
the  Compositae.  These  elements  have  common  origin  in  cambium  cells; 
that  is  to  say,  the  broad  elements  and  the  narrow  sieve-tissue  elements  are 
of  common  descent.  The  broad  cells,  which  later  are  transformed  into 
stereome,  do  not,  therefore,  have  a  distinct  origin.  The  initial  division 
within  the  mother-cell  may  be  periclinal  or  radial,  separating  a  broad  ele- 
ment, destined  to  become  stereomatic,  from  a  similar  one,  which  again 
divides  once  or  twice,  usually  twice,  to  form  the  sieve-tissue  (plate  31, 
fig.  9).  There  is  but  little  difference  in  the  transverse  diameter  of  these, 
the  companion-cells  being  narrowly  fusiform  and  therefore  thickest  at  the 
middle,  while  the  reverse,  of  course,  is  true  of  the  sieve-cells.  The  broad 
elements  are  recognizable  both  by  their  size  and  by  their  more  tenuous 

1  Vuillemin's  description,  "sur  le  dos  des  faisceaux  libe"riens,"  does  not  apply. 
'The  "libriform"  of  Schwendener,  1874. 


Anatomy  and  Histology.  Ill 

protoplasmic  content.  When  they  become  stereomatic  the  first  step  is  the 
great  enlargement  of  their  transverse  diameters,  their  walls  being  thin 
except  at  the  angles,  which  are  thickened  after  the  fashion  of  collenchyma 
(plate  3 1 ,  fig.  8) .  During  this  phase  of  change  the  mutual  pressure  of  the 
developing  stereome  and  the  surrounding  cortex  results  in  the  radial 
flattening  of  the  latter,  and  frequently  in  a  crumpling  of  the  walls  in  the 
stereome.  The  limit  of  the  stereome  may  readily  be  seen  because  of  the 
intercellular  spaces  between  the  cortical  cells  and  those  of  the  stereome. 
Meanwhile  the  sieve-tubes  and  companion-cells  become  displaced  and, 
with  sclerosis  of  the  stereome  elements,  are  destroyed,  and  may  only  with 
difficulty  be  observed  at  all.  Sclerosis  of  the  stereome  proceeds  radially 
from  without  inwardly.  The  compactness  of  the  stereome,  as  also  its 
regularity  and  dimensions,  depends  upon  the  previous  mode  of  growth  of 
the  leptome  as  a  whole,  and  is  therefore  more  irregular  and  of  uneven 
texture,  in  irrigated  plants,  or,  what  amounts  to  the  same  thing,  in  rapidly 
grown  plants.  Sclerosis  also  overtakes  some  of  the  adjacent  leptome 
parenchyma  and,  under  certain  circumstances,  some  of  the  neighboring 
cortical  cells,  but  is  not  preceded  by  their  enlargement. 

The  stereome  in  the  medulla  (plate  3 1 ,  figs.  6,  7)  ,which  has  previously 
been  so  referred  to  for  convenience,  is,  like  the  above-described  leptome- 
stereome,  a  constituent  of  the  mestome  strand.  It  arises  from  elongated 
elements  clustered  about  the  primary  hadrome  elements,  and  is  the  en- 
doxyle  of  Briquet  (1892) ,  but,  in  the  light  of  the  occurrence  of  bicollateral 
bundles  in  the  Chicoriaceae  (Vuillemin,  18840 ;  van  Tieghem,  1884) ,  may  be 
susceptible  of  another  interpretation,  viz,  that  it  represents  the  internal 
leptome  in  these  forms.  This  explanation  is  not  decreased  by  the  very 
close  analogy  between  the  stereome  of  the  leptome  and  of  the  hadrome. 
In  the  young  condition  the  tissue  which  is  destined  to  become  stereome  is 
recognizable  (plate  31,  fig.  6),  in  transverse  section,  by  the  absence  of  in- 
tercellular spaces  and  the  somewhat  thickened  angles,  which,  during  the 
stretching  of  the  walls  previous  to  sclerosis,  become  more  apparent,  as  in 
the  case  of  the  leptome-stereome.  Interspaces  occur  in  the  adjacent  pith 
and  in  the  hadrome  parenchym.  The  tissue,  taking  the  form  of  an  irreg- 
ular lunate  arc  in  transverse  section,  is,  therefore,  while  in  contact  with 
the  hadrome,  not  to  be  referred  to  this  without  careful  consideration. 
The  progress  of  change  into  stereome  is  identical  in  all  respects  with  the 
leptome-stereome,  and  calls  for  no  particular  description;  this  refers  also 
to  the  mutual  displacement  of  tissues  (plate  31,  fig.  7).  The  analogy  to 
the  leptome-stereome  is  strengthened  by  the  circumstance  that  longitu- 
dinal divisions  may  take  place  in  the  earliest  formed  elements,  before  the 
final  complement  of  stereome  cells  is  arrived  at,  though  it  must  be  said 
that  these  divisions  are  not  of  sufficiently  frequent  occurrence  to  enable  one 
to  see  more  than  a  very  few  at  a  time.  The  form  of  the  elements  further 
likens  them  to  the  analogous  ones  in  the  leptome,  being  elongated  and 
having  slightly  inclined  end-walls.  I  am  therefore  inclined  to  regard  the 
medullary  stereome  as  a  tissue  per  se  with  respect  to  the  hadrome,  and  as 
having  much  in  common  with  the  stereome  of  the  leptome,  so  that  it 
would  seem  to  be  properly  regarded  as  representing  the  internal  leptome 
in  genera  of  the  Chicoreaceae 


112  Guayule. 

Precisely  these  relations  occur,  to  all  appearances,  in  certain  of  the 
Boraginaceae,  e.g.,  Symphytum  tuberosum,  Nonnea  alba,  Omphalodia  lini- 
folia,  etc.  (Jodin,  1902).  Concerning  the  leptome,  Jodin  says,  after  speak- 
ing of  the  disappearance  by  crushing  of  the  sieve  and  companion  elements 
("les  primaires  tubes  cribleV'): 

En  meme  temps  que  s'accroissent  les  e'le'ments  libe"riens  primaire's,  on  peut 
assister,  dans  certains  genres  a  un  e"paississement  notable  de  leurs  parois  *  *  *  * 
Dans  d'autres  cas,  cet  6paississement  est  tres  faible  ou  meme  n'a  pas  lieu  *  *  * 
(I.e.,  p.  308.) 

But  no  such  thickening  takes  place  in  the  secondary  leptome.  Appar- 
ently the  thickening  of  which  Jodin  speaks  goes  no  further.  He  does  not 
trace  the  precise  origin  of  the  cells  with  thickened  walls. 

As  to  the  medullary  stereome,  he  says  little,  but  his  figures  show  very 
clearly  the  earlier,  prestereomatic  condition  which  I  have  shown  in  my 
own  figure  (plate  31,  fig.  6).  To  quote  again: 

Nous  aurons  peu  de  choses  a  dire  de  la  moelle;  nous  avons  eu  occasion  de 
parler,  a  propos  des  faisceaux  du  bois,  de  la  zone  pe'rime'dullaire,  et  des  rayons 
me'dullaires.  La  region  me'dullaire  proprement  dite  se  distingue  par  la  taille  de 
ses  cellules  qui  sont  arrondies  en  coupe  transversale,  et  qui  laissent  entre  elles  de 
nombreux  me'ats  triangulaires.  (/.  c.,  p.  322.) 

This  author,  it  is  seen,  points  out  the  same  distinctions  between  the 
perimedullary  zone  and  pith  which  I  have  already  made.  From  this  com- 
parison between  the  guayule  and  the  borages  it  seems  clear  that  we  are 
dealing  with  the  same  behavior,  with  the  very  interesting  distinction  that 
in  the  guayule  the  histological  differentiation  of  the  fibers  proceeds  to 
completion,  while  in  the  plants  studied  by  Jodin  they  are  arrested  in  their 
course  of  development.  This  appears  to  be  connected  with  the  herba- 
ceous character  of  the  stems  in  the  Boraginaceae. 

In  this  connection,  Schwendener's  observations  on  certain  Composi- 
tae  are  of  particular  interest: 

Im  Phloem  der  gr5sseren  Aster  und  Solidago  Formen,  *  *  *  kommen  inner- 
halb  der  starken  primaren  Bastbundel  kleine  secundare  Gruppen  mechanischer 
Zellen  zur  Entwickelung,  welche  zum  Theil  mit  den  kiirzesten  Libriformzellen, 
die  iiberhaupt  vorkommen,  ubereinstimmen,  und  jedenfalls  durchgehend  vom 
typischen  Bast  verschieden  sind.  Die  Lange  diesen  Zellen  variert  zwischen  150- 
300  Mik.;  die  Kiirzesten  erreichen  oft  nur  bis  80  Mik.  Dazu  kommt,  dass  die 
nebeneinander  liegenden  schiefen  Querwande  ahnliche  Zick-zacklinien  bilden,  wie 
sie  sonst  nur  in  kurzzelligen  Libriform  vorzukommen  pflegen.  Bei  Aster  bilden  sie 
im  Querschnitt  netzformige  anastomosirende  Bilden,  zwischen  denen  ein  parenchy- 
matisches  Cambiform  stellenweise  mit  deutlichen  Siebrohren,  eingebettet  liegt.1 

This  distinction  made  by  Schwendener  between  the  sclerosed  element 
of  the  "phloem"  and  typical  bast  applies  throughout  to  Parthenium  ar- 
gentatum.  This  plant,  however,  differs  in  the  distribution  of  the  sclerosed 
elements,  forming  as  they  do  dense  masses  occupying  the  space  previously 
occupied  by  the  whole  of  the  leptome  and  its  associated  libriform. 

Schwendener,  however,  appears  to  assume  the  independent  origin 
of  the  libriform  cells  in  the  leptome,  and  it  is  on  this  point  that  I  advance 
the  view  that  they  have  a  common  origin  with  the  sieve  and  companion 

1  Schwendener,  1874,  p.  152.     I  have  not  had  access  to  this  paper. 


Anatomy  and  Histology.  113 

cells.  After  arriving  at  this  conclusion,  I  found  that  Servettaz  (1909,  p. 
232)  had  already  done  so  with  respect  to  certain  of  the  Eleagnaceae.  The 
close  resemblance  in  the  behavior  of  the  medullary  mass  of  stereome  to 
that  of  the  leptome  forces  criticism  of  this  view  to  the  effect  that  the 
analogy  which  I  have  drawn  is  based  on  the  origin  of  the  stereome  in 
the  hadrome  and  leptome  from  libriform  of  an  identical  mode  of  origin 
on  either  side  of  the  cambium.  This  view,  while  admittedly  possible,  does 
not  agree  with  my  observations,  and  it  is  hoped  that  further  research  will 
bring  evidence  to  light  which  will  show  which  view  is  correct. 

The  secondary  resin-canals,  when  fully  formed,  are  composed  of  an 
endothelium  backed  usually  by  one  row  of  leptome  parenchyma  (plate  29, 
figs.  1,2).  In  transverse  outline,  after  full  development,  they  are  rounded, 
but  gradually  become  compressed  radially  as  they  pass  outward  toward 
the  bark.  The  youngest  ones  measure  upward  of  0.2  mm.  in  tangential 
diameter,  and  grow  in  size  till,  at  the  outer  part  of  the  living  cortex,  they 
may  measure,  in  a  cortex  5  mm.  thick,  over  a  millimeter  tangentially, 
and  with  a  width  a  third  of  this.  The  secreting  cells  undergo  more  or  less 
periclinal  divisions  (with  reference  to  the  axis  of  the  canal),  producing 
sometimes  two  to  three  layers  of  cells  of  endothelial  origin.  The  resin- 
canals  at  length  frequently  become  partly  or  completely  closed  by  an 
ingrowth  of  tissue  (Lloyd,  19086)  of  the  same  character  as  the  cortex  and 
forming  an  interesting  analogy  to  tracheal  plugs  (tyloses).  These  I  call 
pseudoty loses  (plate  32).  The  cells  of  the  pseudoty loses  at  length  become 
filled  with  rubber  and  continue  in  a  living  condition  somewhat  longer 
than  the  surrounding  cortical  tissue,  retaining  their  normal  appearance 
when  the  cortical  cells  toward  the  outside  of  the  stem  have  passed  over 
into  suber.  These  parenchymatous  plugs  are  not  confined  to  the  very  old 
tissues,  but  may  be  found  also  in  young  stems  and  roots,1  though  less 
frequently.  Occasionally  the  medullary  canals,  in  old  plants  at  any  rate, 
become  partially  plugged  in  the  same  manner  (plate  3  2 ,  fig.  3) .  In  addition 
to  these  outgrowths,  resembling  roughly  a  bunch  of  grapes,  one  frequently 
finds  trichome-like  structures,  sometimes  projecting  from  the  walls  and 
also  from  the  plug-tissue  (plate  32,  figs,  i,  6).  Somewhat  similar  appear- 
ances have  been  observed  by  Col,  and  to  this  I  shall  call  attention  again. 
In  this  connection,  however,  I  feel  inclined  not  to  agree  with  this  author 
in  his  criticism  of  Vuillemin,  who  recorded  observing  structures  which  he 
called  "  poils  glanduleux  "  in  the  canals  in  old  rhizomes  of  Arnica  montana 
(Col,  1903,  p.  166).  I  suspect  that  these  "poils  glanduleux"  are  the  same 
structures  as  those  which  I  have  called  pseudotyloses. 

The  pith  undergoes  a  considerable  amount  of  secondary  enlarge- 
ment, so  that  in  a  stem  2.5  cm.  in  diameter,  in  which  it  may  still  be  found 
in  a  living  condition,  its  diameter  is  between  3  and  4  mm.  and  is  irregular 
in  outline.  The  medullary  stereome  does  not  receive  any  secondary  accre- 
tion, but  the  growth  of  the  inner  part  of  the  parenchyma  rays  concomitant 
with  that  of  the  pith,  between  the  edges  of  the  xylem  wedges  and  the 
flanking  stereome,  results  in  the  periclinal  separation  of  these.  Sometimes 
one  may  find  that  cells  near  the  periphery  of  the  pith  have  undergone  a 

1  I  have  observed  them  in  the  primary  canals  (plate  32,  fig.  7). 


114  Guayule. 

rather  regularly  repeated  periclinal  division,  and  the  tissue,  therefore,  has 
much  the  appearance  of  a  cambium.  It  may  also  happen  that  repeated 
divisions  occur  in  a  zone  about  one  of  the  medullary  canals.  The  cause  of 
this  is  not  clear,  though  it  is  possible  that  this  also  is  a  mode  of  growth  of 
the  pith  (plate  42,  fig.  5).  It  does  not  appear  to  be  the  same  as  the  forma- 
tion of  cork,  such  as  I  have  observed  to  occur  following  injury  to  the  pith 
or  adjacent  tissues. 

In  field  plants  normally  neither  pith  nor  parenchyma  rays  (save  a 
very  few  cells)  ever  become  lignified. 

The  wood  in  large  stems  shows  the  usual  distinction  of  alburnum  and 
duramen.  The  latter  is  reddish-brown  in  color,  and  all  the  tracheids  are 
plugged  by  "  Gummipropfen. "  1  Temme  (1885)  and  Ross  (1908)  note 
their  positive  reaction  to  phloroglucin,  which  I  have  verified.  They  are 
very  sharply  confined  to  the  duramen  in  uninjured  stems.  In  one,  2.5  cm. 
in  diameter,  in  which  the  plugs  are  beginning  to  be  formed  with  irregular- 
ity, their  genesis  may  be  followed.  They  first  appear  as  a  thin,  partial  or 
complete  lining,  increasing  irregularly  and  gradually  filling  the  lumen. 
Their  conformation  suggests  the  behavior  of  a  dense  fluid.  Their  positive 
reaction  to  aniline  blue,  which  is  very  marked,  may  indicate  that  they  are 
at  first  similar  to  callus,  but,  as  phloroglucin  shows,  they  later  become 
lignified.  In  the  old  wood  the  plugs  appear  homogeneous,  but  they  stain 
unevenly  with,  e.g.,  Bismarck  brown.  Here  and  there  one  may  note  a 
stratification  in  planes  parallel  to  the  surface  of  the  lumen.  That  resins 
are  absent  from  these  structures  is  shown  by  their  total  failure  to  react  to 
alkanet.  Molisch  (Zimmerman-Humphrey,  1893)  showed  that  gum-plugs 
behave,  with  certain  reagents,  like  lignified  membranes,  but  a  total  par- 
allelism is  denied  by  the  above  reactions.  Lignification  in  any  event 
would  appear  to  be  a  secondary  feature.  Tschirch  (1906,  p.  1180)  identi- 
fies the  substance  as  "bassorin." 

ANNULAR  STRUCTURE. 

The  mature  wood  shows  to  the  naked  eye  an  annular  structure  which 
is  frequently  regarded  as  annual-ring  structure.  In  an  old  stem  what  is 
seen  in  part  is  a  banded  appearance  due  to  differences  in  color  intensity 
(plate  2,  fig.  B) ,  having  no  relation  at  all  to  a  true  annular  structure,  which 
is  readily  seen  under  magnification.  This  is  shown  in  the  two  figures,  one 
of  which  (plate  33,  fig.  i)  was  drawn  to  scale  from  the  inner  alburnum  of 
a  very  old  stem,  and  the  other  (plate  33,  fig.  2)  from  one  a  centimeter  in 
diameter,  showing  ten  rings.  It  is  not  at  all  unlikely  that  these  rings 
represent  ten  years'  growth,  but  this  would  not  justify  the  conclusion  that 
the  rings  are  always  correlated  with  age  in  years.  It  must  not  infrequently 
be  the  case  that  more  than  two  accretions  of  growth  occur  in  response  to 
the  distribution,  in  time,  of  the  rainfall,  and  these  rings,  therefore,  repre- 
sent periods  of  growth  following  rain.  That  these  growth-periods  for  field 
plants  usually  coincide  with  the  summer  seasons  follows  from  the  general 

1  "Wound-gum"  (Temme,  1885)  seems  hardly  a  suitable  term,  since  the 
phenomenon  is  perfectly  normal,  though,  as  will  appear,  the  earlier  secretion  is 
provoked  by  natural  and  by  artificial  wounding.  A  direct  translation  of  Gummi- 
propfen would  be  preferable. 


Anatomy  and  Histology. 


115 


character  of  the  precipitation,  as  elsewhere  described.  The  evidence  gives 
strong  support  to  the  view  expressed  by  Holtermann  (1907)  that  the  ring- 
structure  of  the  wood  is  correlated  with  cessation  and  resumption  of  tran- 
spiration. While  it  is  not  clear  why  an  annular  structure  within  the  annual 
ring  is  present  in  the  wood  of  irrigated  plants,  it  is  quite  possible  that 
it  is  due  to  stimulation  by  successive  irrigations.  These  considerations 
show  that  it  is  practically  very  difficult  to  determine  the  age  of  a  plant 
by  counting  the  rings,  and  this  is  rendered  still  more  so  by  their  short 
radial  measurements.  In  the  case  before  us  (plate  33,  fig.  2)  10  rings 
are  counted  on  a  radius  of  2  mm.,  so  that  the  rings,  taken  altogether, 
have  an  average  thickness  of  0.2  mm.  Excluding  the  innermost  (the  first 
season's  growth)  and  the  deepest,  the  rings  vary  from  0.06  to  0.3  mm. 
approximately.  This,  coupled  with  their  frequently  great  irregularity  and 
indistinctness  (Ross,  1908),  makes  them  difficult  of  recognition. 

The  suggestion  has  been  indicated  inferentially  in  this  connection  by 
Ross  that  the  age  of  a  stem  is  to  be  inferred  from  the  number  of  secondary 
canals  and  stratifications  of  the  secondary  cortex.  A  stem  examined  by 
him,  19  mm.  in  diameter,  showed  eight  zones  of  canals  and  of  alternating 
phloem  layers,  and  he  agrees  with  Endlich  that  the  stem  was  about  ten 
years  old.  I  feel  quite  sure,  however,  that  this  inference  is  far  from  jus- 
tified. For  example,  the  stem  from  which  plate  33,  fig.  2,  was  taken 
was  certainly  over  four  years  of  age,  and  as  certainly  eight  to  ten.  There 
were  only  four  rows  of  secondary  canals.  In  a  stem  with  a  radius  of  i  cm. 
I  count  at  least  ten  cortical  zones,  while  there  are  but  five  in  another  cor- 
tex of  the  same  thickness.  These,  together  with  the  further  fact  that 
under  irrigation  a  seedling  in  five  months  developed  five  rows  of  second- 
ary canals,  show  that  the  number  of  canals  depends  upon  the  rate  of 
growth  and  not  upon  the  number  of  seasons,  and  in  field  plants  the  num- 
ber of  rows  of  canals  is,  roughly,  a  third  to  a  half  less  than  the  age  of  the 
stem  in  years.  A  cortex  5  mm.  thick,  exclusive  of  the  cork,  taken  from  a 
very  old  plant,  about  forty  years  of  age,  shows  about  twenty  canal  zones. 
Some  had  of  course  been  cut  out  by  periderm,  but  scarcely  as  many  as 
twenty. 


TABLE  40.  —  Determinations  by  weight  of  ratios  of  bark  to  wood  infield  plants 
(Whittelsey,  1909). 

Plant. 

Parts  of  the  plant  examined. 

Ratio 
of  bark 
to  wood. 

Plant. 

Parts  of  the  plant  examined. 

Ratio 
of  bark 
to  wood. 

I 
II 

Roots  and  thicker  stems 
f  Roots 

jii 

I  o-79 

0.85 
1-13 

II 

Branches  and  twigs..  .  . 

1.29 
1.63 
1.30 

1:1 

i-9 

2  .  I 

\  Trunk 

116 


Guayule. 


THE  EFFECT  OF  ABUNDANT  WATER  UPON  ANATOMICAL 
STRUCTURE.1 

The  effect  of  irrigation  upon  the  structure  of  the  mature  plant  is  very 
marked.  This  is  especially  noticeable  with  respect  to  the  relative  volumes 
of  the  wood  cylinder  (including  pith  and  medullary  rays)  and  the  "  bark ' ' 
(cortex  and  cork) .  As  this  is  a  question  of  prime  importance  economic- 
ally, it  will  be  treated  first. 

By  means  of  weighing,  Whittelsey  (1909)  determined  that,  in  vari- 
ous portions  of  the  plant,  the  trunks  are  made  up  of  44  to  65  per  cent  bark 
(cortex  and  cork) ,  the  amount  of  bark  being  relatively  larger  in  the  smaller 
twigs.  The  material  was  quite  dry.  (Table  40,  page  115.) 

In  Whittelsey 's  determinations ,  the  pieces  examined  were  first  steamed 
to  render  it  possible  to  separate  the  wood  from  the  cortex.  A  slight 


Fio.  16. — Relative  dimensions  of  wood  cylinder  and  cortex,  wet  and  dry.'in  twigs  of  field  and 
irrigated  plants.     X  20. 

error  is  introduced  by  this  method,  as  some  of  the  resin  exudes  from  the 
cut  ends  of  the  cortex  and  infiltrates  into  the  wood.  This  error  is  appar- 
ent in  table  41 ,  in  which  the  ratios  of  steamed  material  are  smaller  than  in 

TABLE  41. — Ratio  of  bark  to  wood  by  weight  for  field  and  irrigated  shrub,  as  deter- 
mined after  (a)  steaming,  (b)  moist  chamber. 

Two  pieces  each  of  field  and  irrigated  plants  (Cedros,  April  1909):  (a)  Field 
plant  pieces  3.9  to  4.8  mm.  diameter;  (b)  irrigated  plant  pieces,  3.4  to  4  mm.  diam- 
eter. Each  piece  of  (a)  segmented  into  fourths  and  alternate  fourths  placed  in 
each  of  two  lots;  (b)  segmented  into  fourteenths,  similarly  placed  in  each  of  two 
lots.  One  lot  (I)  of  (a)  and  of  (6)  steamed,  de-barked,  dried  in  oven,  and  weighed. 
The  other  lot  (II)  of  each  placed  in  a  moist  chamber  till  fit  for  separating  wood  and 
bark;  then  dried  in  oven  and  weighed. 


Class  of  plant. 

Treatment. 

Weight  of 
wood. 

Weight  of 
bark. 

Ratio. 

(a)  I.  Field  (Cedros). 

Steamed  

gram. 
o    1604 

gram. 
0.2817 

i    71:  + 

(a)  II.  Field  (Cedros)  
(b)  I.  Irrig.  (Cedros)  
(b)  II.  Irrig.  (Cedros)  

Moist  chamber..  . 
Steamed  
Moist  chamber..  . 

0.1663 
0.311 
0.324 

0.  2968 
0.318 
0.3608 

1.78  + 

I  .02 
I  .  II 

1  The  substance  of  what  follows  under  this  caption  was  presented  in  a  paper 
entitled  "The  Responses  of  the  Guayule,  Parthenium  argentatum  Gray,  to  Irriga- 
tion," before  the  Botanical  Society  of  America,  at  its  Boston  meeting,  December 
1909.  (Lloyd,  19106.) 


Anatomy  and  Histology. 


117 


control  material  softened  in  a  moist  chamber.  But  the  small  error,  less 
than  i  per  cent,  is  of  significance  only  when  such  data  are  used  in  large 
calculations. 

From  the  present  point  of  view  a  volumetric  method  is  of  more  value 
than  weighing,  since  the  ratios  derived  by  the  latter  are  disturbed  by  vari- 
ation in  the  specific  gravity ;  but  as  a  comparison 
of  the  ratios  derived  by  both  methods  is  of  use  in 
practice,  they  have  both  been  introduced.     For 
similar  reasons  it  is  im- 
portant   to     know    the 
ratios  derived  from  dry 
material,    and    for   this 
purpose  the  method  of 
displacement  of  alcohol 
has  been  used. 

RELATIVE  VOLUMES 
OF  CORTEX  AND  WOOD. 

The  material  from 
which  the  data  tabulated 
in  table  42  were  obtained 
was  collected  at  Cedros. 
The    irrigated    material 
was   taken   in   September   1908 
from  stems  (fig.  16)  of  two  sea- 
sons' growth.    The  difference  in 
thickness  of  wet  and  dry  cortex 
is  very  slight,  and  is  not  given. 

It  is  seen  that  the  volume 
ratio  of  bark  to  wood  (when  dry) 
in  the  irrigated  plant  is  near  to 
unity  in  the  smaller  twigs  to  0.27 

FIG.  i7.-Relative  thickness  of  cortex  in  stems  of  irri-  in  the  l&TZeT'  UP  tO  *  '^meter 
gated  and  field  plants,  the  wood  cylinders  being  of  13.5  mm.,  beyond  which  no 
of  equal  diameter  when  dry.  •  i  •,  •>•,  T  ^  u 

material  was  available.     In  field 

plants  the  ratio  for  the  smaller  twigs  approaches  2.5,  being  reduced  to 
1.7  for  stems  13  mm.  in  diameter,  and  still  further,  namely,  to  unity,  for 
stems  exceeding  this  diameter  (20  mm.  and  more).  From  the  economic 
point  of  view  this  material  reduction  of  cortical  tissues  in  irrigated  plants 
is  an  important  consideration,  since  it  is  these  tissues  which  contain  the 
rubber. 

The  ratios  for  the  wet  tissues  indicate  the  large  water-holding  capac- 
ity of  the  irrigated  cortex,  especially  as  compared  with  the  field  material. 
These  differences  in  volume  are  quite  obvious  in  the  radial  measurements 
of  the  wood  and  bark.  In  tables  43  and  44  some  accurately  made 
measurements  are  given.  The  ratios  are  illustrated  in  figs.  15,  16.  For 
the  better  direct  comparison  of  field  and  irrigated  plants  dry  twigs  of  the 
same  initial  total  diameter  were  chosen  and  were  measured  both  dry  and 
after  being  soaked  in  water.  The  initial  size  is  shown  in  the  diagrams 


118 


Guayule. 


by  a  half-circle.  To  the  right  of  the  vertical  diameter  is  shown  the  irri- 
gated plant;  to  the  left  of  it  the  field  plant.  The  upper  quadrants  show 
these  when  wet ;  the  lower,  when  dry.  To  be  noted  are  the  greater  capacity 
of  the  wood  cylinder  in  field  plants  for  swelling,  due  to  the  larger  volume 
of  the  parenchyma  rays;  and  the  smaller  capacity  of  the  cortex  tissues  for 
swelling,  due  to  the  larger  rubber-content.  The  greater  volume  of  cork 

TABLE  42. — Volume  of  wood  and  cortex  (Cedros,  Sept.  1908). 


Irrigated  plants. 

Field  plants. 

Diameter 
of  wood 
cylinder. 

Thick- 
ness of 
cortex. 

Ratio  of  cortex 
to  wood  by 
volume. 

Diameter 
of  wood 
cylinder. 

Thick- 
ness of 
cortex. 

Ratio  of  cortex 
to  wood  by 
volume. 

Dry. 

Wet. 

Wet. 

Dry.          Wet. 

Dry. 

Wet. 

Wet. 

Dry. 

Wet. 

mm. 

2  .0 

2-3 

J:i 

7-0 

mm. 
2  .O 
2.4 

J:I 

7-5 
13.0 

i3-5 

mm. 
0-7 
0.8 
i  .  i 
i  .  i 
i  -4 

2  .0 
2  .0 

0-93 
0.81 
0.94 
0-565 
0.43 
0-33 
o.  27 

1-65 

1:11 

mm. 
i-3 
2.7 

i.7 

2-9 

mm. 
0-9 
1-5 

2-33 

2     O 

|:j, 

1.23 
o.7i 

0.6 
0-55 

4-1 
5-9 

6.2 

1:1 

6.6 
14.0 

22  .0 
22.7 

2  .0 
2.8 

3-0 
3-5 
5-o 
4-9 

i   5 
*   7 
i  7 

2.25 

2  .  I 
2.08 
I  .  II 
I  .  II 
1-05 

and  of  the  cortical  intercellular  spaces  in  irrigated  plants  must  also  be 
considered.  As  these  tissues  are  included  in  the  tables  under  the  term 
"bark,"  it  is  obvious  that  an  error  is  introduced  which  is  larger  for  the  irri- 
gated plants.  Hence  the  ratios  ought  to  be,  for  these,  relatively  smaller. 
Table  44  shows  the  same  relations  for  branches  of  larger  size,  in  which 
the  ratios  of  bark  to  wood  are  smaller,  but  relatively  more  so  in  irrigated 
plants.  The  figures  are  of  special  interest,  as  they  include  the  ratio  seen 

TABLE  43. — Transverse  dimensions  of  terminal  twigs  of  irrigated  and  field  plants  of 
the  same  initial  size,  before  and  after  drying  (fig.  15). 


Total 
diameter. 

Diameter 
of  wood. 

Thickness  of 
cortex. 

Thickness  of 
cork. 

Ratio  of  field 
to  irrigated 
cortex,  volume. 

Dry. 

Wet. 

Dry. 

Wet. 

Dry. 

Wet. 

Dry. 

Wet. 

Dry. 

Wet. 

mm. 

mm. 

mm. 

mm. 

mm 

(a)  Irrigated.  . 
(6)  Field  

2.6 
2.6 

3-45 
3-iS 

I.8S 
1-5 

2.0 

i-7 

0-375 
0-55 

0.725 
0.725 

0.13 
O.IO 

0.16 
0.10-0.15 

},,, 

0-95 

(c)  Irrigated.. 
(d)  Field  

3-9 
3-9 

4.«5 
4-5 

3-o 

2-3 

3-i5 

2-5 

0-45 
0.8 

0.925 

I.O 

0.15 

O.IO 

0.19 
0.11-0.15 

H 

1.17 

in  a  plant  from  the  Hacienda  de  San  Isidro,  near  Escalon,  Chihuahua, 
where  guayule  is  said  to  grow  rapidly.  While  the  rate  of  growth  is  not  as 
great  as  supposed,  nevertheless  it  is  sufficiently  so  to  be  reflected  in  the 
structure  of  the  stem,  which  is  intermediate  in  character  between  Cedros 
field  and  irrigated  plants.  In  the  three  cases  the  initial  wood  cylinder 
diameter  (20  mm.)  was  the  same — in  this  way  the  largest  available  sizes 
could  be  compared.  The  thickness  of  the  cork  is,  when  wet,  nearly  the 


Anatomy  and  Histology. 


119 


same  in  all,  though  its  irregularity  makes  accurate  measurement  impos- 
sible. When  dry  it  is  thickest  in  the  Cedros  field  plant,  thinnest  in  the 
Chihuahuan  plant,  and  intermediate  (though  much  more  irregular  in 
thickness)  in  the  Cedros  irrigated  plant.  The  differences  in  the  cortex  are 

TABLE  44. — Comparative  radial  measurements,  in  millimeters,  of  medium-sized  stems 
of  guayule,  wet  and  dry.  Wood  (dry)  cylinder  20  mm.  diam.  in  all.  August 
29,  1909  (fig.  16). 


Dry. 

Wet. 

Ratio  of 

Kind  of  plant. 

Bark 

Cortex 

Bark 

Cortex 

to  wood, 

Wood. 

(cortex 
+cork). 

(without 
cork). 

Wood. 

(cortex 
+cork). 

(without 
cork). 

volume. 

mm. 

mm. 

mm. 

mm. 

mm. 

mm. 

Cedros  irrigated,  3 

years  old  
San    Isidro    (near 

10 

1-7 

1.3 

10.25 

2-5 

2  .0 

0.28 

Escalon)  field  .  .  . 
Cedros  field  

10 
IO 

::35S 

2-35 
3-75 

10.25 
10-37 

3-4 
5-25 

2-95 
4.8 

0.53 
0.89 

The  irregularity  in  the  thickness  of  the  cork  makes  it  difficult  to  measure  it 
properly.     It  is,  at  all  events,  less  than  i  mm.,  and  relatively  thinner  in  field  plants. 

apparent ;  the  index  of  imbibition  of  the  wood  cylinder,  while  still  greater 
in  the  Cedros  field  plants,  is  relatively  much  smaller  than  in  smaller  stems, 
because  of  the  compression  of  the  medullary  rays. 

Tables  45  and  46  are  based  upon  comparative  measurements  of  Cedros 
field  and  irrigated  plants.     The  latter  material,  however,  was  collected  in 

TABLE  45. — Relative  amount  of  bark  and  wood  in  guayule,  by  volume  (dry).    Irrigated 
plants  (Cedros,  Apr.  1909). 


No. 

Total 
diameter. 

Diameter 
of  wood. 

Thickness 
of  bark. 

Ratio  of 
volume  of  bark 
to  wood. 

mm. 

mm. 

mm. 

i 

15-°' 

I3.0 

I  .0 

-32 

2 

9.13 

7-32 

0.9 

-48 

3 

8-3 

6.7 

0.8 

•50 

4 

5-8 

4-25 

0.77 

•79 

5 

5-6 

4-35 

0.62 

.70 

6 

5.22 

3-75 

0-735 

.00 

7 

3-5 

2.6 

0.45 

.00 

8 

2.4 

i  -4 

0-5 

.46 

9 

2-15 

1-4 

0-37 

.  ii 

IO 

1-52 

o-95 

0.29 

.67 

ii 

21.7 

18.6 

1-55 

•3i 

12 

12.4 

10.9 

0-75 

-40 

Nos.  i  to  10,  stems  from  single  plant  taken  Apr.  1909.     No.  n,  stem  from 
different  plant  taken  Sept.  1908.    This  is  the  trunk  (main)  brought  from  Saltillo. 

No.  12,  root  from  plant'take'n  Sept.  1908.    No.  8,  base  of  piece  through  crowded 
nodes,  bottom  of  1909  growth,  hence  bark  a  little  thicker  here. 

April  1 909.  At  this  time  growth  had  only  just  recommenced,  from  which  it 
is  evident  that  the  amount  of  water  received  between  September  1908 
(at  which  time  I  left  Cedros)  and  the  time  of  my  later  visit  was  very 
small — and  the  information  obtained  showed  this  to  have  been  the  case. 
The  chief  value  of  these  tables,  besides  indicating  somewhat  more 
fully  the  points  already  made,  lies  in  the  evidence  they  bear  that  the  ratio 


120 


Guayule. 


of  bark  to  wood  has  increased  during  the  period  between  the  dates  given 
above,  as  shown  in  brief  in  table  47. 

As  will  be  seen,  this  change  in  volume  in  the  irrigated  cortex  is  to  be 
referred  chiefly  to  an  increase  in  the  rubber-content. 

TABLE  46. — Relative  amount  of  bark  and  wood  in  guayule,  by  volume  (dry).   Field 
plants  (Sept.  1908). 


No. 

Total 
diameter. 

Diameter 
of  wood. 

Thick- 
ness of 
bark. 

Ratio  of 
vol.  of  bark 
to  wood. 

No. 

Total 
diameter. 

Diameter 
of  wood. 

Thick- 
ness of 
bark. 

Ratio  of 
vol.  of  bark 
to  wood  . 

mm. 

mm. 

mm. 

mm. 

mm. 

mm. 

I 

29.25 

20.25 

4-5. 

I  .  II 

11 

4.67 

2.7 

0.99 

.0 

3 

9.0 

•45 
6.7 

I.IS 

0.81      i 

13 

4-2 

2-75 

0.72 

•25 

4 

15.45 

10-5 

2-47 

.11      ' 

14 

4.0 

2-5 

0-75 

-47 

5 

9-25 

5-93 

i-7 

•37 

15 

3-5 

2-45 

0.52 

•31 

6 

7-85 

5-0 

1  -4 

.44 

16 

3-35 

2-25 

0-55 

.4 

7 

5.8 

3-9 

0-95 

•34 

17 

2.6 

1  -5 

0-55 

•25 

8 

5-25 

3-3 

0.97 

•45 

18 

1.9 

i  .42 

0.24 

.00 

9 

5-15 

3-i5 

I  .0 

•57 

19 

24.2 

19.0 

2.6 

0.63 

10 

4-9 

3-12 

0.89 

-63    | 

20 

iQ-75 

7-7 

i-5 

0.80 

No.  i,  basal  portion  of  trunk;  No.  2,  a  lateral  root;  No.  3,  a  tap-root;  Nos.  4 
to  1 8,  series  from  a  single  plant;  No.  19,  San  Isidro,  Chihuahua,  large  plant;  No.  20, 
Chihuahua,  trunk,  seedling. 

A  discrepancy  which  always  appears  between  the  ratios  by  weight, 
not  here  given,  and  by  volume  is  greater  for  the  irrigated  plants  on  ac- 
count of  the  larger  rubber- content  of  the  wood  cylinder  in  the  field  plants. 

The  smaller  ratios  for  the  smaller  twigs  (Nos.  1 2  to  1 8  inclusive)  for 
field  plants  in  table  46  as  compared  with  those  in  table  42  are  due  in 
part  to  the  fact  that  the  material  was  collected  in  the  height  of  the  grow- 
ing-season, and  hence  the  new  growths  have  something  of  the  character 
of  the  irrigated  plant  in  its  low  rubber-content  especially.  The  impor- 
tance of  this  fact  in  its  relation  to  practice  is  shown  elsewhere. 

TABLE  47. — Ratios  of  bark  to  wood  in  Cedros  irrigated  plants  collected  in 
September  1908  and  in  April  1909. 


Date. 

Range  in  size  of  stem 
(dry)  diameter. 

Limits  of  ratios  for 
these  sizes. 

Sept.  1908  (table  42)  
Apr.  1909  (table  45)  

3-4  to'g.S 
3  •  5  to  5  .  8 
3-5  to  8.3 

0.93  to  0.43 
i  .00  to  0.79 
i  .  oo  to  o  .  5 

It  is  interesting  to  note,  further,  that  the  ratios  for  root-tissues  are 
similar  in  both  types  of  plant,  but  are  smaller  in  irrigated  plants.  They 
are  also  much  smaller  than  those  for  the  stem  in  each  type.  This  fact 
should  be  considered  in  making  up  averages  to  indicate  the  relative  eco- 
nomic value  of  cultivated  guayule.  The  roots,  in  proper  practice,  should 
not  enter  into  a  calculation  for  returns  in  manufacture,  and  by  excluding 
them  the  average  ratios  (those,  namely,  for  stems  only)  are  higher,  but 
relatively  higher  for  field  plants,  as  shown  in  the  tables  above. 


Anatomy  and  Histology.  121 

EFFECT  OF  VARIOUS  AMOUNTS  OF  WATER  OF  IRRIGATION. 

The  most  fundamental  economic  question  for  which  an  answer  will  be 
sought  in  these  pages  is  that  relating  to  the  production  of  rubber  by  plants 
under  irrigation.  As  bearing  upon  the  answer  is  the  relation  of  the  above 
tissue-responses  to  the  amount  of  water  supplied,  as  already  indicated  in 
table  44.  That  the  inference  based  upon  the  data  there  displayed  is  cor- 
rect is  indicated  by  the  measurements  taken  from  irrigated  plants  from 
two  localities  where  the  conditions  were  unavoidably  and  markedly  dif- 
ferent, as  follows: 

Cedros. — Stocks  planted  March  1907,  by  Mr.  C.  T.  Andrews.  Irri- 
gated freely  till  April  1908.  Went  dry  till  summer  rains.  25  cm.  growth 
during  1907  and  during  1908.  Long  drought  from  September  1908  to 
May  1909,  but  probably  irrigated  somewhat  during  this  period.  Sample 
plant  collected  May  10,  1909  (plate  17,  fig.  B). 

Caopas. — Whole  plants  of  medium  size  planted  January  1908,  and 
abundantly  irrigated  till  June  1908.  On  account  of  failure  to  start  in 
January  they  were  trimmed  back  down  to  the  stouter  branches.  New 
shoots  then  started,  these  being  for  the  chief  part  included  above  under 
"total  diameter  3  mm.  or  less."  The  Caopas  presa  broke  out  in  June 
1908,  so  that  between  that  date  and  the  time  of  collection  (May  9,  1909) 
no  irrigation  was  possible  (plate  46,  fig  B). 

TABLE  48. — Comparison  of  ratios  of  bark  to  wood  by  weight  for  plants  from 
Cedros  and  Caopas,  irrigated. 


Parts. 

Cedros. 

Caopas. 

Stems,  3.0  mm.  diameter  

1.16 
o  80 

1.56 

Larger,  up  to  23  mm.  diameter  (wood  cylinder)  

0-525 

0.84 

From  the  figures  it  may  fairly  be  concluded  that  the  amount  of  dis- 
turbance in  rate  of  growth  in  the  tissues  considered  is,  within  certain  wide 
limits,  related  to  the  amount  of  available  soil-water.  The  less  the  water, 
the  thicker  the  bark  (cortex) ,  and  vice  versa.  The  Caopas  plants  certainly 
had  less  water  than  the  Cedros  plants,  and  the  ratios  of  bark  to  wood 
stand  in  these  at  1.56  and  1.16,  respectively,  for  the  small  branches  which 
grew  in  both  plants  under  irrigation.  As  to  the  reduction  in  radial  meas- 
urement of  the  chief  rubber-bearing  tissue,  the  cortex,  it  must  be  remem- 
bered that  this  is  compensated  for  by  the  more  rapid  growth  of  plants 
under  irrigation  (up  to  six  times) ,  so  that  the  absolute  amount  of  cortical 
tissue  in  an  irrigated  plant  will  be  greater  than  that  in  a  field  plant  for  the 
same  period  of  growth. 

The  rate  of  growth  determines  the  total  volume.  In  order  to  obtain 
an  empirical  factor  for  the  purpose  of  conveying  to  the  mind  an  approxi- 
mate notion  of  the  relative  ability  of  field  and  irrigated  plants  to  produce 
"bark"  in  a  given  period  of  time,  I  took  two  average  twigs  of  one  season's 
growth,  removed  the  leaves,  decorticated,  and  weighted.  The  figures  in 
table  49  were  obtained.  Here  it  is  evident  that,  aside  from  the  much 
more  rapid  growth  in  weight  in  irrigated  plants,  the  amount  of  rubber- 


122 


Guayule. 


TABLE  49. 


Ratio  of 

Source. 

Weight 
of  cortex. 

cortex  of 
irrigated  to 

field  plant. 

grams. 

Irrigated  plant.  . 

5-44 

\          6 

Field  plant  

0-97 

[ 

bearing  tissue  formed  by  a  single  twig  is  at  least  5.5  times  that  produced 
on  a  field  twig  of  similar  age. 

By  introducing  this  factor,  and  that  of  rapid  growth,  into  the  calcula- 
tion, it  may  be  seen  that  the  resulting  total  volume  of  the  rubber-bearing 

tissue  preponderates  in  the  most 
rapidly  grown  plants,  and  from  the 
data  set  forth  there  emerges  the 
conclusion  that  it  is  possible  to  reg- 
ulate irrigation,  and  thereby  to  pre- 
determine, within  the  usual  limits 
approximately  shown  in  the  preced- 
ing pages,  the  relative  total  amount 
of  cortex  and  wood.  I  do  not  forget 
that  difficulties  of  another  sort, 

related  to  the  manufacture  of  crude  rubber,  may  be  introduced,  but  with 
these  we  are  not  at  present  concerned.  The  remaining  part  of  the  ques- 
tion, as  to  the  amount  of  rubber  the  tissues  of  irrigated  plants  are  capable 
of  producing,  is  in  part  answered  beyond,  in  a  succeeding  chapter. 

EFFECT  OF  DROUGHT  FOLLOWING  IRRIGATION. 
From  the  ecological  point  of  view,  it  seems  reasonable  to  argue  that 
the  greater  production  of  parenchymatous  tissues  is  in  the  direction  of  suc- 
culency,  and  is  an  adaptive  response  to  the  arid  conditions  under  which 
the  plant  lives.  The  largest  growth  of  these  tissues  is  found  in  the  paren- 
chyma rays  as  well  as  in  the  cortex,  and  there  can  be  little  doubt  that  the 
parenchyma  of  the  pith,  parenchyma  rays,  and  cortex  function  to  some 
degree  of  efficiency  as  water-storage  reservoirs.  It  is,  however,  clear  from 
the  measurements  which  have  been  presented  in  the  foregoing  tables  that 
the  way  in  which  this  succulency  works  is  not  by  capacity  for  a  large 
amount  of  water — irrigated  plants  are  superior  in  this  respect — but,  it 
must  be  argued,  in  holding  it  more  tenaciously.  The  efficiency  in  this 
direction  is,  however,  not  very  great,  if  we  measure  it  crudely,  as  when  we 
observe  the  rate  of  wilting  when  the  plant  is  removed  from  the  ground, 
and  it  is  not  in  any  sense  to  be  compared  with  the  resistance  of  desert  suc- 
culents in  this  regard.  What  the  rubber  itself  may  contribute  to  this  mod- 
erate efficiency  can  be  answered  only  in  speculative  fashion.  The  death 
of  large  numbers  of  plants  scattered  over  large  areas  after  severe  drought 
does  not  warrant  extravagant  notions,  at  any  rate. 

EFFECT  OF  IRRIGATION  ON  THE  PHYSICAL  CHARACTERS  OF 
THE  WOOD. 

The  wood  of  irrigated  plants  is  noticeably  harder  and  more  rigid  than 
that  of  field  plants  (Lloyd,  19086) .  This  is  apparent  upon  cutting  or  upon 
twisting  or  bending.  For  the  purpose  of  measuring  the  differences  in  flex- 
ural  rigidity,  two  slender  wood  cylinders  of  equal  (2  mm.)  diameter  were 
obtained  by  freeing  them  from  the  cortical  tissues,  and  were  then  sub- 
jected to  bending  before  and  after  drying.  It  was  found  that,  when  still 
wet,  the  wood  of  the  irrigated  plant  is  three  times  more  rigid  than  that  of  a 
field  plant  (the  exact  ratio  was  1 1  to  3.5)  and  when  dry  the  ratio  is  2  to  i. 


Anatomy  and  Histology.  123 

This  mechanical  difference  appears  to  be  due  to  the  nature  and 
extent  of  the  medullary  rays  and  their  relation  to  the  wood,  together  with 
the  relative  amount  of  mechanical  tissue  in  the  latter.  The  very  great 
difference  in  the  size  of  the  parenchyma  rays  is  seen  in  both  transverse 
and  tangential  sections,  as  shown  in  the  figures  (plate  28,  figs.  1,2;  plate 
33,  figs.  3  to  6),  in  which  it  is  seen  that  the  rays  in  field  plants  are  very 
much  larger  than  in  irrigated  plants.  For  this  reason  alone  we  must  con- 
clude, other  things  equal,  that  the  former  would  be  much  the  less  rigid. 
Further,  the  walls  of  the  medullary-ray  cells  in  irrigated  plants  become 
much  thickened  and  lignified  (plate  33,  fig.  7),  while  in  field  plants  the 
cells  remain  thin-walled  indefinitely,  with  the  exception  that  there  occur 
among  them  a  very  few  tracheid-like  cells  (plate  33,  fig.  8),  with  very 
peculiarly  thickened  walls.  These  are  so  few  in  number  that  it  is  difficult 
to  attach  any  physiological  or  mechanical  importance  to  them.  The 
mechanical  elements  of  the  wood  in  the  irrigated  plants  appear  more 
compact  to  the  eye,  the  lacunae  being  smaller  and  the  whole  mass  being 
made  up  of  smaller  and  more  regularly  developed  cells.  It  may  here  be 
remarked,  also,  that  the  development  of  medullary  stereome  is  somewhat 
stronger,  but  this  scarcely  contributes  a  measurable  quantity  to  the  total 
rigidity  of  a  stem  more  than  i  to  2  mm.  in  diameter. 

The  vessels  of  irrigated  wood  are  frequently  plugged  with  the  so- 
called  "  Gummipropfen "  at  an  age  of  two  years  or  even  less.  Their 
appearance  is  hastened  by  artificial  or  by  natural  wounding,  as  the  dying 
back  of  the  peduncle.  The  pith-cells  may  undergo  a  considerable  amount 
of  sclerosis,  without  change  of  shape.  The  lumen  is  frequently  very  much 
reduced  in  size,  and  the  walls  are  traversed  by  delicate  branching  canali- 
culi  (plate  29,  figs.  5,6).  Sclerosis  of  pith-cells  occurs  in  Manihot  glaziovii 
near  the  leaf-bases,  that  is,  at  the  nodes  (Calvert  and  Boodle,  1887),  in  the 
pith  of  Liriodendron  tulipifera  (Holm,  19090),  and  in  that  of  Cornus  flor- 
ida  (Holm,  19096).  The  sclerosed  cells  of  the  last-named  are  identical  in 
structure  with  those  of  the  guayule,  both  as  regards  the  pores  and  the 
small  size  of  the  lumen.  Jodin  (1902)  also  notes  a  total  sclerosis  of  the 
pith  in  Cynoglossum  officinale,  and  partial  in  Lithospermum  fruticosum. 
The  sclerosis  of  pith-cells  under  irrigation  suggests  the  value  of  experimen- 
tation with  other  plants  in  the  reverse  direction. 


124  Guayule. 


THE  PEDUNCLE. 

It  has  been  said  elsewhere  that  the  method  of  branching  is  correlated 
with  the  production  of  the  inflorescences,  the  terminal  new  branches  in 
twos  or  threes  being  produced  by  the  outgrowth  of  the  uppermost  axillary 
buds  below  the  peduncle.  There  is  but  little  secondary  thickening  in  the 
peduncle,  while,  correlated  with  its  slender  character,  there  is  a  large  de- 
velopment of  mechanical  tissue  (plate  34,  fig.  i) .  In  the  young  condition 
the  chief  points  of  difference  between  a  peduncle  and  a  definitive  foliage 
stem  are  the  absence  of  medullary  canals1  and  the  interruption  of  the 
hypodermis  by  the  development  of  chlorenchyma,  without,  however,  the 
reduction  of  the  collenchymatic  character.  There  are  about  6  to  8  of 
these  longitudinal  chlorophyll  strips,  provided  with  numerous  stomata 
with  their  axes  placed  longitudinally,  as  is  usual.  The  cortex,  which  is,  of 
course,  primary,  has  usually  6  resin-canals  above  to  10  below.  There  are 
about  a  dozen  bundles,  and  these,  before  secondary  thickening  is  con- 
cluded, produce  a  few  tracheids,  though  tracheae  are  equally  prominent 
constituents  of  the  hadrome.  A  weak  interfascicular  cambium  is  formed, 
but  its  cells,  without  losing  their  cambial  character,  undergo  sclerosis,  pre- 
serving their  rectangular  transverse  section.  Stereome  strands  are  formed , 
as  in  the  stem,  just  within  the  pericycle  and  in  the  pith,  but  their  relative 
amount  of  development  is  here  much  greater.  It  spreads  toward  the 
interfascicular  cambium,  involving  the  parenchyma-ray  cells,  sometimes 
entirely.  The  stereome  within  the  cambium  thus  unites  to  form  a  complete 
stereomatic  sheath,  or  perhaps  is  occasionally  interrupted  by  incomplete 
sclerosis  of  parenchyma  rays.  Outside  of  the  cambium,  the  sheath  is 
interrupted  by  the  cortical  rays,  inasmuch  as  the  cells  here  do  not  become 
sclerotic,  except  a  few  adjacent  to  the  leptome-stereome.  The  non-scle- 
rosis of  the  interfascicular  cortex  comports  with  the  view  of  the  chiefly 
leptomatic  origin  of  the  stereome.  The  small  amount  of  cortical  stereome 
adjacent  to  the  leptome-stereome  may  readily  be  recognized  both  by  the 
color  and  shape  of  the  cells. 

In  addition  to  the  normal  stereome,  as  this  may  be  called,  some  of  the 
pith-cells,  a  few  of  the  outer  cortical  cells,  and  some  also  from  the  collen- 
chyma  become  stereomatic.  The  chief  part  of  this  adjunctive  stereome  is 
derived  from  the  pith,  which  contributes  a  notable  amount  to  the  inner 
surface  of  the  mechanical  sheath.-  In  the  basal  part  of  the  peduncle  a 
periderm  occurs,  giving  rise  to  a  layer  of  cork  about  o.i  mm.  thick.  No 
absciss  tissue  is  formed,  and  the  dead  peduncle  persists  for  years  until 
disintegration  finally  overtakes  it.  As  death  extends  toward  the  base  of 
the  peduncle,  the  vessels  both  of  the  peduncular  wood  and  that  of  the 
adjacent  stem  become  plugged,  as  elsewhere  described.  It  is  a  matter  of 
interest  to  note  that  the  structure  of  the  peduncle  is  very  similar  to  that  of 
the  stem  in  the  mariola  (Parthenium  incanum) ,  and  bears  notable  resem- 
blances also  to  the  herbaceous  stems  of  P.  hysterophorus  and  P.  lyratum. 

1  I  have  seen  one  canal  in  the  pith  on  one  occasion. 


Anatomy  and  Histology.  125 

THE  LEAF. 
COTYLEDONS. 

The  cotyledons  of  the  guayule  are  dorsiventral  (plate  34,  figs.  7  to  9). 
Both  surfaces  are  free  from  trichomes.  The  parenchyma  is  composed  of 
six  layers  of  cells,  of  which  the  upper  two  form  a  palisade  tissue.  The 
spongy  parenchyma  is  not  highly  differentiated,  for  the  two  lower  layers 
of  cells  only  have  distinctive  characters,  and  these  are  not  pronounced.1 
There  are  no  resin-canals  in  the  blade,  though  the  four  primary  canals  of 
the  hypocotyl  pass,  two  into  each  of  the  petioles.  The  mid-vein  (plate 
31 ,  fig.  i)  in  the  petiole  is  composed  of  two  mestome  strands,  the  origin  of 
which  has  already  been  discussed,  but  is  single  above,  and  there  are  two 
lateral  veins.  The  cotyledons  show  certain  well-marked  responses  to 
water  and  light  conditions.  The  cotyledons  of  seedlings  grown  in  the 
shade  are  slightly  thinner  than  those  of  field  seedlings,  and  have  a  much 
larger  superficies  (plate  34,  figs.  6,  9).  All  the  parenchyma  cells,  and  all 
the  epidermal  cells  save  the  guard-cells,  are  expanded  in  directions  parallel 
to  the  surface.  The  epidermal  cells  of  the  surface  are  deeper  also.  The 
intercellular  spaces  of  the  spongy  parenchyma  are  more  extensive,  and 
the  cell-walls  are  thinner.  These  changes  are  in  accord  with  observations 
in  general,  but  it  is  of  importance  to  note  that  the  internal  and  external 
adjustments  of  the  cotyledon  are  produced  by  changes  in  shape  of  the 
cells,  and  not  by  change  in  number  of  cells.  This  is  well  illustrated  by 
the  behavior  of  the  epidermis,  both  as  to  the  shape  of  the  cells  and  the 
number  of  stomata.  One  might  well  suppose  that  there  would  be  an 
increase  in  the  number  of  stomata,  as  well  as  in  their  size,  in  plants  well 
supplied  with  water  and  not  subjected  to  severe  aerial  conditions.  Counts 
of  the  stomata  per  unit  of  surface  gave  the  results  shown  in  table  50. 

That  is,  the  number  of  stomata  per  unit  of  area  appears  to  depend  on 
the  amount  of  growth  of  the  leaf.  The  greater  number  on  the  lower 
surface  in  the  field  plant  is  due  to  the  rolled-leaf  effect,  which  is  absent 
from  the  shade  plant.  The  result  is  that,  in  the  plant  which  has  to  con- 
serve water,  there  are  relatively  more  stomata  to  carry  it  off.2  Evidently 
therefore,  the  supposedly  adaptive  adjust- 
ments as  regards  the  stomata  do  not  involve 


Shade. 


Field. 


100 
130 


their  numbers  in  plants  of  the  same  species  Surface, 

under  different  conditions.  The  thinner  and 
less  strongly  cuticularized  epidermis  of  the  Upper  . 
shade  plant  may  indeed  compensate,  as  may  ,  Lowrer  •  • 
also  the  more  extensive  intercellular  system, 
for  the  relatively  fewer  stomata.  But  inasmuch  as  the  dampering  of  trans- 
piration by  stomata  is  not  effective  within  wide  limits  (Lloyd,  19080),  such 
differences  in  numbers  as  the  above  may  be  of  little  or  no  significance.3 

1  The  structure  is  very  similar  to  that  of  the  cotyledon  of  Helianthus,  in 
which,  however,  the  intercellular  spaces  are  relatively  more  extensive,  and  there 
are  more  layers  of  cells. 

2  Transpiration  rate  is  greater  per  unit  of  surface  in  sun  plants  than  in  shade 
plants  (Bergen,  1908;  Sampson  &  Allen,  1909). 

3  On  this  question  the  student  should  consult  Renner,  1910. 


126  Guayule. 

That  the  structural  adjustments  of  the  cotyledon  involve  only  change 
in  shape  of  the  cells  is  shown  also  by  the  responses  of  seedlings  grown  in 
a  soil  of  high  osmotic  equivalent  (plate  34,  figs.  5,8).  Under  this  condi- 
tion the  surface  of  the  cotyledon  is  greater  than  in  the  field  plant  (plate  34, 
figs.  4,  7),  but  its  thickness  is  also  much  greater.  The  cells  of  the  paren- 
chyma are  correspondingly  deeper  (plate  34,  fig.  8),  their  extension  of  size 
being  at  right  angles  to  the  leaf-surface  and  parallel  to  the  direction  of 
greater  extension  of  its  size.  This  cotyledon  represents  the  maximum 
response  in  a  xerophytic  direction,  and  it  is  worthy  of  note  that  under 
normal  field  conditions  this  response  does  not  ensue,  indicating  that 
succulency  here  is  primarily  the  effect  of  a  soil  condition,  namely,  the  low 
physiological  availability  of  the  soil- water. 

PROPHYLLS. 

The  earliest  foliage  leaves  (prophylls)  show  a  slight  advance  toward 
the  bifacial  condition,  though  normally  they  are  dorsiventral.  Neverthe- 
less it  is  possible  to  induce  a  marked  bifacial  condition  by  growing  seed- 
lings in  soil  which  contains  a  very  meager  supply  of  water  (plate  35,  figs.  5, 
8).  Such  plants  grow  very  slowly  indeed,  and  the  earliest  foliage  leaves 
attain  but  a  small  size  and  are  relatively  thick,  while  the  resin-canals  are 
of  greater  diameter.  The  extreme  departure  from  this  condition  is  shown 
by  shade-grown  leaves  (plate  35,  figs,  i,  2),  with  the  greater  superficial  ex- 
tent of  which  the  shape  and  dimensions  of  their  cells  are  correlated,  while 
field  seedlings  and  those  grown  in  soil  of  high  osmotic  equivalent  are  very 
similar  in  structure.  To  be  noted,  however,  both  in  these  leaves  and  in 
the  cotyledons,  is  the  behavior  of  the  spongy  parenchyma.  The  lower- 
most layer  of  cells  shows  this  especially.  In  shade  plants  (plate  35,  fig.  i) 
the  cells  are  broad,  as  viewed  in  a  transverse  section,  and  dumbbell- 
shaped.  In  the  field  plant  (plate  35,  figs.  3,  4)  they  are  columnar  and 
have  two  spaces  between  each  two  cells.  In  the  seedlings  exposed  to  dis- 
tinctly unfavorable  soil  these  spaces  are  almost,  or  frequently  entirely, 
absent  (plate  35,  figs.  6,  7). 

THE  DEFINITIVE  LEAF. 

Although  the  foliage  leaves  exhibit  a  structural  advance  over  the 
cotyledons,  it  is  noticeable  that,  as  compared  with  these,  the  definitive 
foliage  leaves  exhibit  a  smaller  range  of  response.  These  have,  to  be  sure, 
a  dense  clothing  of  trichomes,  described  elsewhere,  and  this  fact  may 
explain  the  difference,  which  receives  no  elucidation  in  the  character  of 
the  stomata  (plate  35,  fig.  n).  These  show  no  special  so-called  adaptive 
features.  There  can  be  no  doubt  that  closely  packed  hairs  form  an  effective 
insulation  which  has  the  effect  of  producing  mesophytic  conditions,  so  to 
speak,  over  the  leaf-surface,  both  by  dampering  transpiration  and  by 
modifying  the  sunlight.1 

To  determine  the  extremes  of  structural  response  within  the  leaf,  I 
have  taken  one  leaf  from  an  irrigated  plant  during  the  period  of  rapid 
growth  and  one  from  a  large  seedling  of  good  size  after  a  six  months' 

lCf.  Wiegand's  interesting  paper  of  1910. 


Anatomy  and  Histology.  127 

drought,  as,  presumably,  final  resultants  of  a  complete  antithesis  of  soil- 
water  conditions.  In  these  one  observes  a  thinner  leaf,  though  slightly 
more  strongly  cutinized,  in  the  field  plant  (plate  35,  fig.  13),  otherwise  but 
little  difference  is  to  be  seen.  In  both  the  structure  is  strongly  isolateral, 
with  six  layers  of  cells  in  each,  but  one  may  detect  a  somewhat  more  exten- 
sive system  of  intercellular  spaces  in  the  irrigated  plant  (plate  35,  fig.  15), 
though  it  must  be  said  that  the  difference  appears  but  slight.  The  canals 
show  no  appreciable  difference.  Neither  the  stomata  nor  the  substoma- 
tal  spaces  afford  any  ground  for  special  comment,  while  the  outer  epider- 
mal walls  are,  contrary  to  expectation,  slightly  thicker  in  irrigated  plants. 
A  denser  trichome  covering  in  the  field  plant  may  indeed  compensate  for 
this,  but  the  observed  differences  are  very  slight. 

It  remains  possible  that  differences  in  the  character  of  the  soil  in 
which  these  plants  grew  are  responsible  for  the  close  similarity,  but  the 
rate  of  growth  in  the  irrigated  plants  and  their  resulting  general  mesophy- 
tic  character  minimize  the  value  of  the  supposition. 


128  Guayule. 


ABBREVIATIONS  USED  IN  PLATES    22  TO  39. 

pc.  pericambium.  cot.  cotyledon. 

end.  endodermis.  cor.  cortex. 

ex.  exodermis.  r.c.  resin-canal. 

/,,/2.  primary  and  secondary  leptome.  ped.  peduncle. 

hi,h2.  primary  and  secondary  hadrome.  pr.  parenchyma-ray  (or  cells  of  this) . 

s.  stereome.  lib.  libriform. 

m.c.t.  median  cotyledonary  trace.  /.  lacuna. 

l.c.t.  lateral  cotyledonary  trace.  Ip.  leptome. 

c.t.  cauline  traces.  pd.  periderm. 

c.t.l.  cauline  traces,  first  primordial  leaf. 

DESCRIPTION  OF  PLATE  22. 

1-5.  Development  of  endodermal  canals  of  root.     Fig.  3,  root  8  mm.  diameter. 

6.  Lateral  growth  of  endodermal  cells  and  intercalated  walls,  with  Caspary's 

band.     Root  0.6  mm.  in  diameter. 

7.  Cortex   (with  exodermis)  of   a  tap-root  0.46  mm.  in  diameter.     First  peri- 

dermal  divisions  in  the  hypodermal  cells. 

8.  The  reduction  of  cortical  cells  into  cork  has  reached  the  endodermis. 

9.  Portion  of  a  tap-root  showing  early  cambium  divisions. 

10.  The  same  in  a  somewhat  older  root  to  show  development  of  intercalated 

mestome  strand  (h2,  12)  at  outer  edge  of  primary  hadrome  plate  (/tt). 

11.  A  still  later  stage  showing  closure  of  secondary  hadrome  about  the  parenchyma 

island  by  uniting  with  intercalated  mestome  strands.  The  separation  of 
these  from  the  primary  plate  is  due  to  disturbance  by  the  growth  of  a  second- 
ary root. 

12.  Primary  leptome  in  contact  with  the  endodermis,  in  the  hypocotyl. 

13.  A  secondary  cortical  canal  arising  in  the  phloem.     The  meatus  is  just  at  this 

moment  appearing. 

14.  Details  of  secondary  leptome,  showing  the  relation  of  the  resin-canal  to  the 

remaining  elements.     Root  4  mm.  in  diameter. 

15.  Pores  in  walls  of  endodermal  and  cortical  cells  (cf.  plate  41,  fig.  2). 

1 6.  Pericambium  in  a  root  after  primary  stereome  has  begun  to  develop. 


LLOYD 


PLATE  22 


130  Guayule. 


DESCRIPTION  OF  PLATE  23. 

1-7.  Tap-root. 

1.  Field  seedling. 

2.  Irrigated  seedling. 

3-5.   Field  seedling,  2  mm.  diameter  (cf.  plate  40,  figs.  2,  3). 

3.  Sector  to  show  arrangement  of  tissues  and  distribution  of  rubber. 

4.  Endodermis  and  pericambium  of  a  root  1.2  mm.  in  diameter. 

5.  Same,  thickened  and  compressed ;  root  1.5  mm.  in  diameter;  globules  of  rubber. 

6.  Region  just  within  primary  canals.    Primary  leptome-stereome  (sj  ;  secondary 

leptome-stereome  (s2). 

7.  Parenchyma  ray  enlarged  to  show  globules  of  rubber. 

8.  Epidermis  of  the  hypocotyl,  first  peridermal  divisions. 

9.  Trichome  from  hypocotyl. 


132  Guayule. 


DESCRIPTION  OF  PLATE  24. 

i.  Cotyledon  en  face,  to  show  distribution  of  vascular  strands. 
2-5.  Sections  through  cotyledonary  collar  and  upper  part  of  hypocotyl  in  an 
ascending  series. 

6.  Section  through  cotyledonary  collar,  and  lower  part  of  first  internode. 

7.  Section  through  first  node  of  epicotyl. 

8.  Section  through  stem  near  second  node  of  epicotyl. 

9-11.  Section  through  different  levels  to  show  primitive  trachea  and  its  relations 
to  older  elements. 

9.  Base  of  hypocotyl. 

10.  Middle  of  hypocotyl. 

11.  Base  of  cotyledonary  collar. 

12.  Origin  of  cauline  strands  at  a  level  between  those  of  figs.  4  and  5,  but  in  a 

younger  specimen. 

13.  Diagram  of  the  vascular  tissue  (hadrome)  in  the  plantlet.    Arrows  indicate 

fusion;  dotted  line  the  primary  trachea. 


LLOYD 


134  Guayule. 


DESCRIPTION  OP  PLATE  25. 

1-6.  Primary  endodermal  resin-canals  of  the  hypocotyl. 

1.  Cell  lineage  (diagrammatic). 

2.  A  not  infrequent  but  abnormal  behavior. 

3.  Definitive  condition,  as  regards  cell- walls. 

4.  Definitive  condition  after  growth  and  readjustment. 

5.  Endodermic  stereids  adjacent  to  canals. 

6.  Stereids  (pericyclic  ?)  just  within  canal  in  stele. 
7-10.  Transverse  sections;   hypocotyl. 

7.  Field  seedling,  1.8  mm.  diameter. 

8.  Irrigated  seedling  of  slow  growth.    Secondary  splitting  apart  of  wood  cylinder. 

9.  Etiolated  seedling  2  mm.  diameter. 

10.  Etiolated  seedling  2  mm.  diameter.  Hypocotyl  through  cotyledonary  collar. 
Secondary  hadrome  of  cotyledonary  median  traces  is  morphologically 
cauline  and  distinct  from  primary  tissue.  /.  c.  t.,  Lateral  cotyledonary 
trace  passing  out  from  the  stele. 

ioa,  1 06.     Cotyledonary,  median  traces  at  another  level. 


PLATE  25 


I0b 


10 


136  Guayule. 


DESCRIPTION  OF  PLATE  26. 

1.  Hypocotyl  of  an  etiolated  seedling:  the  stele,  showing  rupture  of  wood  col- 

umn and  secondary  opening  of  medullary  rays. 

2-4.  Sectors  of  the  hadrome  to  show  relative  amount  of  conductive  and  mechanical 
tissue. 

2.  Field  seedling. 

3.  Etiolated  seedling. 

4.  Irrigated  seedling  of  slow  growth. 

5-7.  The  mechanical  tissue  (libriform)  of  hadrome  of  above. 

5.  Field  seedling. 

6.  Etiolated  seedling. 

7.  Irrigated  seedling. 

S-io.  Stereome  (leptome)  of  above. 

8.  Field  seedling. 

9.  Etiolated  seedling. 

10.  Irrigated  seedling. 

1 1 .  Protohadrome  of  a  field  seedling ;  the  size  of  the  lacunae. 

12.  Same;  base  of  a  peduncle,  irrigated 

13.  Same;  an  irrigated  plant,  2  cm.  from  apex. 


oOoo 
oooo 


o 


138  Guayule. 


DESCRIPTION  OF  PLATE  27. 

1.  Hypocotyl,  irrigated  seedling  of  very  rapid  growth,  about  2  months  old. 

2.  Wood  of  irrigated  stem  6.5  mm.  in  diameter. 

3.  Wood  of  field  stem  8  mm.  in  diameter.    (Figs.  2  and  3  are  drawn  to  same  scale.) 

4.  An  irrigated  stem. 

5.  Growth  of  1907,  terminal  twig  of  a  very  large  field  plant.     (Figs.  4  and  5  are 

drawn  to  same  scale.) 

6.  Irrigated  seedling  of  very  slow  growth. 

7.  Irrigated  seedling  of  very  rapid  growth. 

8.  Field  seedling.     (Figs.  6  to  8  are  drawn  to  same  scale;  the  total  wood  to  the 

scale  shown,  libriform  to  a  larger  scale.) 

9.  Base  of  growth  of  1908,  Cedros,  irrigated  plant,  i  year's  growth. 

10.  2  years'  growth  (1906-07),  field  plant,  Cedros.   (Figs.  9  to  10  are  drawn  to  same 
scale.) 


140  Guayule. 


DESCRIPTION  OF  PLATE  28. 

1.  Irrigated  plant,  stem  i  year's  growth. 

2.  Field  plant,  stem  2  years'  growth.     Note  width  of  parenchyma  rays  and  depth 

of  cortex.     (Figs,  i  and  2  are  drawn  to  same  scale.) 

3 .  Hypocotyl,  field  seedling  less  than  i  year  old. 

4.  Cortex  of  hypocotyl  after  secondary  enlargement.     Transverse  section. 

5.  Cortex  of  root;  tangential  section. 


142  Guayule. 


DESCRIPTION  OF  PLATE  29. 

1.  Leptome  region  of  hypocotyl  shown  in  fig.  3. 

2.  Leptome  region  of  hypocotyl  shown  in  fig.  4. 

3.  Hypocotyl,  irrigated  seedling  of  very  rapid  growth. 

4.  Hypocotyl,  irrigated  seedling  of  very  slow  growth. 

5.  Pith,  Cedros  irrigated  stem,  showing  sclerosed  pith-cells. 

6.  A  few  of  these  cells  in  detail. 


PLATE  29 


144  Guayule. 


DESCRIPTION  OF  PLATE  30. 

1.  Hypocotyl,  field  seedling  for  station  2,  April  1909,  less  than  8  months  old. 

2.  First  internode,  epicotyl;  the  primary  stereome  bundles. 

3.  Periderm  opposite  bundle  i  in  fig.  2. 

4.  Periderm  opposite  bundle  2  in  fig.  2. 
5-11.  Trichomes. 


LLOYD 


PLATE  30 


10 


146  Guayule. 


DESCRIPTION  OF  PLATE  31. 

1 .  Transverse  section  through  upper  part  of  petiole  of  a  well-matured  cotyledon, 

in  which  one  sees  the  ends  of  the  resin-canals,  rclt  rc2.    Above  this  level  they 
end  blindly. 

2.  A  section  of  one  of  the  resin-canals  (rc^  in  fig.  i,  somewhat  nearer  base  of  same 

cotyledon. 

3.  Transverse  section  3  mm.  from  apex  of  stem  of  a  field  plant,     co,  cork.     Slow 

growth . 

4.  Transverse  section  through  a  stem  in  rapid  growth  2  mm.  below  apex.     The 

five  medullary  canals  are  established  according  to  a  2/5  phyllotaxy. 

5.  A  section  through  a  stem  apex  above  that  of  fig.  4,  in  which  the  order  of  de- 

velopment of  the  cortical  canals  is  seen  to  relate  to  that  of  the  leaves. 

6.  Inner  part  of  hadrome  bundle  of  stem,  showing  cells  which  become  stereids. 

7.  The  same,  in  which  the  stereids  are  of  maximum  size  and  their  walls  partially 

thickened.      (Figs.  6  and  7  are  drawn  to  the  same  scale.) 

8.  Enlargement  of  stereid  elements  in  leptome  previous  to  thickening  of  walls. 

9.  The  leptome  in  which  the  primordial  cells  which  become  stereids,  st,  are  seen. 

10.  A  very  young  medullary  resin-canal  in  the  secretory  cells  of  which  are  seen 

relatively  large  globules  of  rubber.    Minute  ones  appear  in  adjacent  cortical 
cells. 

1 1 .  One  of  these  cortical  cells  on  a  larger  scale,  to  show  the  rubber  granules  more 

exactly. 

12.  Secretory  cell  of  medullary  resin-canal  after  periclinal  divisions,  showing  gran- 

ules of  rubber. 

13.  The  schizogenous  origin  of  the  medullary  canal. 

14.  Peridermal  divisions  in  the  collenchyma. 


LLOYD 


PLATE  31 


148  Guayule. 


DESCRIPTION  OF  PLATE  32. 

i ,  6.  Sections  of  resin-canals  in  which  trichome-like  structures  occurred. 

2.  Transverse  section  through  a  20-year-old  cortex. 

3.  Pith-canal  with  pseudotylose. 

4.  Primary  cortical  canal  of  the  stem  with  pseudotylose ;  the  spread  of  peri  derm 

(pd.)  about  a  stereome  bundle  is  shown. 

5.  Pseudotylose  in  an  old  cortical  canal. 

6.  Trichome-like  columns  formed  in  pith  in  seedlings  of  slow  growth. 

7.  Primary  (endodermal)  root  canal  with  pseudotylose. 


PLATE  32 


150  Guayule. 


DESCRIPTION  OF  PLATE  33. 

1.  Annual  rings  in  duramen  of  an  old  stem. 

2.  Annual  rings  in  the  wood  of  an  8-year-old  stem. 

3-4.  Parenchyma  rays  as  seen  in  a  tangential  section  of  a  stem  of  a  field  plant. 

5-6.  Same,  irrigated  plant. 

7.  Sclerosed  parenchyma-ray  cells  of  an  irrigated  plant. 

8.  Tracheidal  parenchyma-ray  cell  of  a  field  plant. 

9.  Libriform  of  an  irrigated  plant;  old  wood. 
10.  Same,  field  plant;  old  wood. 


PLATE  33 


152  Guayule. 


DESCRIPTION  OP  PLATE  34. 

1.  Transverse  section  of  a  peduncle.    The  mechanical  tissues  are  hatched. 

2.  Lower  epidermis  of  the  cotyledon  of  a  field  seedling.  To  accompany  figs. 

4  and  7. 

3.  Same,  seedling  grown  in  shade  (to  accompany  figs.  6  and  9). 

4,  7.  Cotyledon  of  a  field  seedling. 

5,  8.  Cotyledon  of  a  seedling  grown  in  soil  with  a  high  saline  content. 

6,  9.  Same,  grown  with  abundant  water  and  shade. 


LLOYD 


PLATE  34 


154  Guayide. 


DESCRIPTION  OF  PLATE  35. 

1-2.  First  primordial  leaf,  seedling  grown  in  shade. 

3-4.  Same,  field. 

5-8.  Same,  seedling  under  irrigation  (exp.  141,  May  1908). 

6-7.  Same,  in  strongly  saline  soil. 

9.  Epidermis  of  a  field  plant  (Station  2). 

10.  Same,  irrigated  plant. 

11.  Stoma  of  leaf,  figs.  6-7. 

12.  Transverse  section,  definitive  leaf;   field  (Station  2,  April  1909) 

13.  Same,  irrigated.     August  1908. 

14.  Detail  of  fig.  12. 

15.  Detail  of  fig.  13. 


LLOYD 


PLATE  35 


156  Guayule. 


DESCRIPTION  OF  PLATE  36. 

1.  Transverse  section,  lower  end  of  a  tap-root  after  considerable  secondary  thick- 

ening, to  show  large  size  of  primary  canals. 

2.  Same,  diarch  secondary  root. 

3.  Detail,  to  show  thin  secondary  cortex  and  secondary  canals. 

4.  Triarch  secondary  root. 

5.  Stem  of  slow  growth.    The  large  size  of  the  cortical  canals  is  notable. 

6.  Diagram  of  the  medullary  canals  at  the  apex  of  a  slowly  growing  stem. 

7.  Early  anastomosis  of  primary  cortical  canals,  parallel  to  the  plane  of  the 

cotyledons. 

8.  Same,  at  right  angles  to  the  plane  of  the  cotyledons. 


LLOYD 


PLATE  36 


158  Guayule. 


DESCRIPTION  OF  PLATE  37. 

1-5.  Primary  cortical  canals. 

1.  Endodermal  origin  in  a  young  epicotyl.     This  canal  has  beqn  rotated,  but  not 

displaced  otherwise. 

2.  Etiolated  seedling-epicotyl.    Lateral  displacement,  but  not  sufficient  to  mask 

its  relation  to  the  endoderm. 

3.  Displacement  sufficient  to  mask  origin. 

4-5.  First  internode.  An  earlier  and  later  stage  in  the  derivation  of  the  cortical 
canal  from  the  endodermis.  The  endodermal  origin  is  masked  by  indefinite 
character  of  endodermis. 

6-7.  Later  and  earlier  conditions  of  ventral  foliar  canal,    xy  indicates  the  position 

of  the  hadrome  of  the  mid-vein. 
8.  Cortical  canal  in  Parthenium  incanum.     The  endodermal  origin  is  clear. 


LLOYD 


PLATE  37 


160  Guayule. 


DESCRIPTION  OF  PLATE  38. 

1.  Transverse  section,  definitive  stem,  to  show  primary  plan  of  canals. 

2.  Petiole,  to  show  the  canals  of  a  large  foliage  leaf.     Dorsal  canals  of  cauline 

origin.     Ventral  canals,  foliar. 
3-5.  First  foliage  leaf. 
6-9.  Another  (first)  foliage  leaf. 
10-18.  Definitive  leaf. 


LLOYD 


PLATE  38 


11 


162  Guayule. 

. 


DESCRIPTION  OF  PLATE  39. 

i.  Chief  (foliage)  shoot  and  axillary  bud.     Medullary  canals  in  both. 
2-3.   Successive  planes  through   chief  peduncular  shoot,  and  axillary  bud,  field 

plant.     All  canals  pass  into  bud. 

4-7.   Successive   planes  from  below  upwards  of   an   irrigated   chief  (peduncular) 
shoot  and  axillary  bud.    Two  medullary  canals  which  have  passed  into  the 
bud  branch  to  form  four  canals. 
8.  Secondary  cortical  resin-canals  in  old  cortex.     Tangential  section. 


LLOYD 


PLATE  39 


CHAPTER  VI. 

THE  RESIN-CANALS  IN  THE  GUAYULE.1 

THE  CANAL-SYSTEMS. 

Because  of  the  comparative  interest  of  the  facts  involved  it  is  here 
proposed  to  summarize  my  observations  on  the  origin,  structure,  and  dis- 
tribution of  the  resin-canals  in  Parthenium  argentatum.  The  canals  occur 
in  this  plant  in  well-defined  systems,2  as  follows: 

(a)  Primary  systems: 

1.  In  the  cotyledons,  the  hypocotyl,  and  the  root,  a  continuous* 

system. 

2.  Independent  of  this,  the  systems  in  roots  of  secondary  and 

higher  order. 

3.  In  the  cortex  of  the  stem  and  in  the  dorsal  moiety  of  the 

leaves,  forming  a  continuous  system. 

4.  An  independent4  system  in  the  dorsal  moiety  of  the  leaf. 

5.  An  independent4  system  in  the  ventral  moiety  of  the  leaf. 

6.  In  the  pith  of  the  stem:  the  medullary  system. 
(6)  Secondary  systems: 

i.  Recurrently  in  the  secondary  leptome  of  the  root  and  stem, 
forming  continuous  concentric  systems.  There  are  no 
transverse  anastomoses  between  the  several  concentric 
systems,  such  as  occur  in  a  laticiferous  plant,  Manihot 
glaziovii,  according  to  Calvert  and  Boodle  (I.e.}. 

PRIMARY  CANALS  IN  THE  ROOT  AND  HYPOCOTYL. 

These  have  their  origin  in  the  endodermis  and  are  included  within 
it,  as  shown  for  many  Compositse  by  Vuillemin,  van  Tieghem,  Col  (I.e.)  ,and 
Holm  (1908). 

To  be  noted  is  a  formation  of  a  band  of  Caspary  in  the  new  walls  aris- 
ing in  the  cells  destined  to  become  a  part  of  the  canal.  In  the  root  there 
are  two  groups,  one  group  of  two  to  four  (or  occasionally  six)  canals  op- 
posite each  primary  phloem  bundle  (plate  36,  fig.  i).  While  this  grouping 
is  generally  true  for  the  Tubuliflorae,  the  number  of  canals  varies,  e.g.,  in 

1  For  a  summary  of  the  knowledge  of  the  resin  or  oil  canals  in  the  Compositae 
up  to  1903,  see  Col  (1903).    An  excellent  historical  sketch  of  the  development  of 
our  knowledge  of  organs  of  secretion  of  oil,  resin,  etc.,  is  given  by  Tschirch  (1906) 
at  p.  1095. 

2  In  the  usual  sense  as  employed  by,  e.g.,  Vuillemin  (18846),  and  by  Calvert 
and  Boodle  (1887). 

3  Vuillemin  (18846)  properly  pointed  out  the  independence  of  the  canals  of 
the  hypocotyl  and  epicotyl.    He  says:  "les  systemes  se"cr6teurs  des  deux  membres 
ou  des  regions  differentes  de  mfime  membre  sont  toujours  distinctes." 

4  As  to  origin. 

165 


166  Guayule. 

Silybum.  Col  states  that  there  are  six  in  each  group,  and  it  appears  from 
his  account  that  the  number  of  those  which  pass  into  the  hypocotyl  is 
scarcely  reduced.1  In  Parthenium  argentatum,  however,  the  number  of 
primary  canals  is  usually  not  more  than  four ;  hence  it  appears  that  in  the 
transition  zone  the  number  of  canals  may  be  doubled.  The  four  pri- 
mary canals  of  the  hypocotyl  pass  into  the  petioles  of  the  cotyledons  in 
pairs,  there  to  end  blindly  (plate  31,  figs,  i  and  2).  They  do  not  reach  as 
far  as  the  blade. 

The  absence  of  canals  in  the  blade  of  the  cotyledons  is  to  be  noted. 
According  to  Vuillemin,  the  more  numerous  canals  in  the  hypocotyl  of 
Calendula  officinalis  pass  (in  part?)  into  the  cotyledons,  on  which  point 
Col  takes  issue.  Col's  figure  of  the  seedling  of  this  species  shows  groups 
of  canals  opposite  four  epicotyledonary  bundles,  and  these  he  identifies 
with  the  hypocotyledonary  canals,  and  shows  none  in  association  with 
the  paired  median-trace  bundles  of  the  cotyledons.  The  position  in  which 
Col's  drawing  shows  the  canals  suggests  that  they  may  be  the  lower  ends 
of  the  epicotyledonary  canals.  In  many  cases,  indeed,  the  true  hypoco- 
tyledonary canals  may  not  follow  the  primary  median  bundles  even  into 
the  petioles  on  the  cotyledons,  while  in  other  cases  they  may.  They  may, 
therefore,  end  blindly  in  the  hypocotyl,  by  a  morphological  recedence 
which  Col  has  cleverly  traced  for  the  plant  as  a  whole  by  his  extended 
comparative  study  of  numerous  Compositae.  In  Parthenium  argentatum 
there  are  no  other  canals  in  the  cotyledons  (plate  34,  figs.  4  to  6). 

PRIMARY  CORTICAL  CANALS. 
IN  SECONDARY  ROOTS. 

Primary  cortical  canals  in  secondary  roots  and  in  those  of  higher 
orders  arise  de  novo  from  the  endodermis  of  the  new  member.  This  is 
brought  about  by  the  morphological  independence  of  the  endodermis  in 
the  roots  of  different  order.  Secondary  roots  are  not  infrequently  triarch 
(plate  36,  fig.  4),  and  have  then  three  groups  of  canals,  two  to  four  in 
each  group.  In  roots,  either  primary  or  of  a  higher  order,  which  grow 
chiefly  in  length,  the  canals  attain  relatively  large  transverse  dimensions, 
and,  with  a  lacunation  of  the  septae  between  them,  there  arise  columns  of 
cells  connecting  the  tangential  walls  (plate  36,  fig.  3).  The  interpreta- 
tion has  been  properly  applied  by  Col  (I.e.,  p.  166)  to  similar  appearances 
in  Solidago.  Col's  observations  do  not,  however,  negative  Vuillemin's 
previous  conclusions,  "dans  les  vieux  rhizomes  d' Arnica  montana,  etc.," 
as  I  point  out  elsewhere. 

IN  THE  EPICOTYL  AND  DEFINITIVE  STEM. 

As  one  ascends  the  axis  the  endodermis  becomes,  as  is  usually  the 
case,  a  less  definite  structure.  For  this  reason  it  becomes  increasingly 
difficult  to  determine  with  precision  the  exact  origin  of  the  primary  corti- 

1  Vuillemin  (18840)  notes  in  Silybum  a  reduction  in  the  number  of  endodermal 
root-canals  by  ending  blindly,  so  that  a  reduced  number  pass  through  the  hypo- 
cotyl into  the  cotyledons.  The  question  naturally  arises  whether  the  reduction 
in  number  is  not  produced  by  coalescence,  as  in  guayule. 


The  Resin-Canals  in  the  Guayule.  167 

cal  canals.  At  the  level  at  which  the  earliest  canals  appear,  namely,  im- 
mediately above  the  level  of  the  cotyledonary  node,  the  difficulty  is  not 
as  great  as  higher  up.  Here  the  endodermis  is  evidently  involved,  and  it 
seems  conclusive  that  the  whole  of  the  canal  structure  is  derived  from  it, 
though  the  cell  lineage  is  not  as  evident  even  in  a  young  condition  as  it  is 
at  higher  levels  in  Parthenium  incanum.  This  at  any  rate  accords  with 
previous  observations,1  and  is  without  any  doubt  the  case  in  those  parts 
of  the  stem  where  the  endodermis  is  regular  enough  to  display  its  morpho- 
logical relations.  I  therefore  conclude  that,  were  it  possible  to  follow  the 
development  of  the  structure,  it  would  be  found,  even  in  the  higher  parts 
of  the  stem  in  Parthenium  argentatum,  where  the  endodermis  is  quite  ill- 
defined,  to  have  originated  in  this. 

The  course  of  development  is  as  follows:  A  tangential  division  takes 
place  in  one,  or  it  may  be  two  or  three  neighboring  endodermal  cells.  In 
the  cell  destined  to  give  rise  to  the  canal  a  radial2  division  crosses  the  first 
wall  so  as  to  form  four  cells,  realizing  the  "division  crucial"  of  van  Tieg- 
hem.  Periclinal  divisions,  however,  take  place,  cutting  off  special  secre- 
tory cells,  four  in  number,  from  a  tier  of  supporting  cells,  while  these  suffer 
a  still  further  subdivision.  Two  pairs  arising  from  the  inner  two  cells  of 
the  original  four  are  cut  off,  and  are,  so  to  speak,  discarded  from  the  canal 
structure,  as  occurs  also  in  the  primary  root-canals.  Only  the  outer  cells 
of  the  outer  original  two  become  divided,  so  that  fourteen  cells  in  all  arise, 
of  which  four  are  the  original  secretory  cells,  six  are  the  supporting 
cells,  and  four,  or  perhaps  six,  excluded — this  in  the  mariola,  Parthenium 
incanum  (plate  37,  fig.  8). 

The  canals  of  guayule  (plate  37,  figs.  1-5)  bear  sufficient  resemblance 
to  those  of  the  mariola,  so  that  it  would  be  unsafe  to  deny  their  entirely 
endodermal  origin.  Secondary  changes,  by  which  the  number  of  secre- 
tory cells  as  well  as  that  of  the  supporting  cells  is  multiplied,  need  not 
be  described,  as  they  consist  only  of  repeated  radial  divisions  and  some- 
times of  tangential  ones  in  the  secreting  cells. 

These  canals  suffer  more  or  less  displacement  (plate  37,  fig.  3)  accord- 
ing to  circumstances,  often  sufficient  to  mask  their  origin.  For  this  reason 
they  have  been  alluded  to  as  cortical  by  Ross  (1908)  and  by  Fron  and 
Francois  (1901),  without  raising  the  question  as  to  their  origin.  This  is, 
perhaps,  the  reason  that,  although  Col  (1903)  asserts  the  endodermal 
origin  of  the  cortical  canals  in  the  Tubuliflorae,  his  drawings  sometimes 
fail  to  show  clearly  this  derivation,  as,  e.g.,  in  Aster  astivalis. 

In  Anthemis  mixta  and  Lasthenia  glabrata,  however,  the  origin  is 
clearly  shown,  and  it  seems  that  the  canals  are  less  elaborately  organized 
than  in  Parthenium  and  suffer  less  displacement.  My  own  effort  has  been 
to  show  conclusively  the  origin  of  these  canals,  with  the  result  that  the 
work,  in  part  of  Vuillemin,  of  van  Tieghem,  and  of  Col,  is  supported. 

1  Van  Tieghem  (1884)  insists  correctly  upon  the  endodermal  origin  of  the 
primary  canals,  but  I   am  unable  to  recognize  the  distinction  between  canals 

hordes"  and  "non-horde's,"  though,  correlated  with  the  more  definite  character 
of  the  endodermis  in  the  roots,  the  canals  are  here  more  regular  and  somewhat 
simpler  in  their  structure  (but  certainly  not  "non-horde's")  than  in  the  stem. 

2  With  respect  to  the  stem. 


168  Guayule. 

TOPOGRAPHICAL  RELATION  OF  CORTICAL  CANALS. 

The  canals  of  endodermal  origin,  instead  of  taking  a  cortical  position, 
may,  in  various  plants,  take  a  position  within  the  pericycle  alternating 
with  the  bundles,  or  opposite  the  bundles  between  the  leptome  and  the 
endodermis.  Holm  finds  such  canals  in  Ambrosia  artemisicefolia,  though 
such  was  supposed  to  be  the  case  for  A.  trifida  only  (Vuillemin,  van  Tieg- 
hem).  In  Eupatorium  (Holm,  1908),  also,  canals  occur  "outside  the  lep- 
tome." The  displacement  of  the  canals  and  accompanying  cells  of  the 
endodermis  to  a  position  nearer  the  axis  appears  to  have  led  Vuillemin  to 
draw  the  conclusion  that  the  endodermis  of  the  stem  is  superposed  on 
that  of  the  hypocotyl,  an  inference  which,  as  Dangeard  (1889,  p.  122)  has 
said,  needs  confirmation.  Vuillemin's  figure  (I.e.,  p.  191)  is  susceptible  of 
a  different  interpretation. 

In  the  young  epicotyledonary  axis  in  Parthenium  incanum,  the  canals 
of  the  cortex  are  more  usually  arranged  in  pairs,  flanking  the  median  leaf- 
traces.  This  is  the  permanent  arrangement,  as,  e.g.,  also  in  Zinnia  (Vuil- 
lemin) and  in  Olearia  haasii  (Col,  1903).  It  comes  about,  therefore,  that 
in  the  definitive  stem  of  this  plant  the  cortical  canals  are  alternate  in  posi- 
tion with  the  bundles.  In  the  guayule  they  are  usually  placed  on  the  same 
radius  with  the  bundles,  and  stand  therefore  opposite  the  leptome.  Both 
of  these  arrangements  occur  in  various  Compositae.1 

The  transition  from  an  alternating  position  of  the  canals  with  respect 
to  the  bundles  in  the  epicotyl  to  the  radially  opposite  position  presents 
an  ontogenetic  summary  of  these  two  conditions  characterizing  various 
Compositae  in  which  one  or  the  other  arrangement  occurs.  It  may  be 
added,  however,  that  the  position  opposite  the  bundle  in  guayule  is  not 
invariable;  exceptionally,  canals  occur  opposite  medullary  rays.  This  is 
true  of  both  primary  and  secondary  cortical  canals,  though  Ross  states 
the  contrary. 

ANASTOMOSIS. 

Anastomosis  and  branching  frequently  occur  between  the  canals  of 
the  primary  cortical  system.  The  four  earliest -formed  epicotyledonary 
canals,  which  arise  in  pairs  associated  with  the  first  and  second  primordial 
leaf-traces,  are  connected,  each  with  the  other  canal  of  each  pair,  by  a  trans- 
verse meatus,  which  lies  above  the  level  at  which  the  lateral  cotyledonary 
traces  pass  out  from  the  axis.  This  transverse  meatus  is  a  prominent  fea- 
ture of  the  epicotyl,  and  is  frequently  the  starting-point  of  several,  usually 
four,  canals.  Anastomoses  in  the  definitive  stem  are  usually  to  be  found 
at  the  nodes,  and  in  stems  with  very  short  internodes  they  are  frequently 
quite  numerous.  For  this  reason,  in  part,  the  number  of  primary  cortical 
canals  seen  in  a  transverse  section  varies,  as  stated  by  Ross  (1908).  As 
the  stem  thickens  (aside  from  secondary  changes)  the  number  of  canals 
increases, so  that  from  5  to  20, approximately, may  be  seen  (plate  36, fig.  5). 

1  There  are  very  few  cortical  canals  in  Parthenium  lyratum  and  in  P.  hyster- 
ophorus,  and  they  occur  on  one  or  both  sides  of  a  bundle,  but  not  opposite  to  it. 
Neither  do  they  stand  opposite  a  medullary  ray,  strictly  speaking,  though  this 
appears  to  be  the  case  in  P.  incanum.  In  P.  arctium  Bartlett  they  are  numerous 
and  alternate  with  the  bundles. 


The  Resin-Canals  in  the  Guayule.  169 

MEDULLARY  CANALS. 
IN  THE  EPICOTYL. 

All  medullary  canals  are  protogenic.  Secondary  ones  do  not  occur. 
The  typical  number  of  canals  is  not  established  for  ten  or  more  internodes, 
this  probably  being  variable.  In  field  seedlings,  or  ones  of  slow  growth, 
the  distance  from  the  cotyledonary  to  the  tenth  node  is  very  short,  and 
the  particular  behavior  of  the  canals  is  difficult  to  determine.  Etiolated 
seedlings,  therefore,  throw  more  light  on  the  matter,  though  it  can  not  be 
asserted  that  the  behavior  in  such  is  always  normal,  e.g.,  when  the  canals 
end  blindly  above,  as  they  have  been  observed  to  do,  instead  of  continuing 
to  the  apex  of  the  stem.  These  short  canals  may,  perhaps,  be  regarded  as 
"poches  secreteurs" — the  pockets  in  which  Col  sees  reduced  canals.  The 
following  notes,  based  upon  a  series  of  sections  made  of  a  seedling  about  10 
cm.  tall,  with  16  nodes,  show  that  the  definitive  condition  is  established 
only  at  length,  even  the  sixteenth  node  being  sometimes  reached  before 
the  full  complement  of  canals  occurs. 

No  canals  below  the  fifth  node. 

At  fifth  node,  one  canal  passing  into  bud. 

Fifth  internode,  lower  part,  no  canals;  upper  part,  four  canals. 

Sixth  node,  one  of  these  into  bud.  One  branches,  making  four  enter- 
ing lower  part  of  sixth  internode. 

Upper  part  of  sixth  internode,  two  canals ;  higher  up,  three,  one  send- 
ing a  branch  to  bud  of  the  seventh  node. 

Seventh  node.  At  this  level  two  more  canals,  making  five  to  enter 
the 

Seventh  internode,  in  which  one  ends,  leaving  four  in  middle  part. 

Eighth  node,  four  canals,  of  which  one  branches  into  bud. 

Eighth  internode,  two  canals  in  middle  part.  One  branches,  making 
three  to  the 

Ninth  node,  at  which  the  bud  receives  a  branch. 

Tenth  node,  four,  one  branching  to  bud.  All  but  one  end  blindly,  so 
that  the 

Tenth  internode  receives  only  one  canal.  Two  more  arise,  making 
three  for  the 

Eleventh  node.  One  passes  without  branching  into  the  bud,  leaving 
two  to  enter  the 

Eleventh  internode.    One  of  these  ends,  so  that  one  canal  reaches  the 

Twelfth  node,  at  which  one  more  arises  by  branching,  and  enters  the 
bud. 

Twelfth  internode  receives  one,  which  ends  blindly  on  reaching  the 

Thirteenth  node,  where  a  new  one  arises  and  passes  into  the  bud. 

Thirteenth  internode  has  no  canals  in  the  lower  part. 

Fourteenth  node,  one  canal  arises  and  passes  into  the  bud. 

Fifteenth  and  sixteenth  internodes,  two  canals  in  each. 

Despite  the  irregularity  in  numbers,  and  also  in  position,  it  is  clear 
that  the  canals  in  the  pith  have  peculiar  relations  with  the  nodes.  When 
one  arises  it  does  so  in  connection  with  the  development  of  an  axillary  bud, 
and  either  enters  it  or  sends  a  branch  to  it.  This  is  to  be  inferred  also 


170  Guayule. 

from  the  regular  occurrence  of  live  canals,  the  primary  number  in  the 
growing  stem  apex.  In  seedlings  with  short  internodes  the  canals  appear, 
of  course,  nearer  the  hypocotyl.  In  a  field  seedling  3  cm.  tall,  with  two 
dozen  or  more  nodes,  I  found  one  canal  at  5  mm.  above  the  hypocotyl. 
The  next  section  cut  had  one.  Similarly  in  an  irrigated  seedling  with 
short  internodes. 

The  absence  of  pith-canals  in  the  epicotyl  suggests  a  primitive  al- 
liance with  those  tubuliflorous  forms  in  which  canals  are  entirely  absent 
from  the  pith. 

IN  THE  DEFINITIVE  STEM. 

At  the  growing  apex  within  0.5  mm.  one  finds  a  strictly  primary  ar- 
rangement of  these  canals.  There  are  five,  one  opposite  each  orthosti- 
chy.1  In  a  slowly  growing  stem,  however,  in  which  the  nodes  are  crowded 
upon  each  other,  through  frequent  branching  and  anastomosis,  the  num- 
ber seen  will  vary  usually  between  three  and  six.  The  union  and  separa- 
tion of  the  canals  is  associated  with  the  formation  of  large  lacunae  giving 
off  large  passages  of  irregular  shape,  but  on  the  whole  running  longitudi- 
nally. In  a  single  section,  therefore,  one  may  count  as  many  as  a  dozen 
canals,  and  nearby  as  few  as  three  or  four.  In  rapidly  growing  shoots  the 
anastomoses  and  branches  are  not  so  apparent,  though  they  occur  here 
also.  From  the  canal  nearest  to  it  each  bud  receives  normally  a  single 
branch,  which,  itself  branching  after  entering  the  bud,  increases  till  the 
complement  is  reached.  Pith-canals  do  not  enter  the  leaf. 

TOPOGRAPHIC  RELATIONS  OF  MEDULLARY  CANALS,     jj 

Although  the  primary  number  of  pith-canals  is  more  or  less  masked 
by  branching  and  anastomosis,  as  already  mentioned,  a  study  of  the  on- 
togeny of  the  stem  can  not  fail  to  show  that  five  is  the  primary  number 
(plate  38,  fig.  i) ,  and  further,  that  they  arise  in  the  same  order  as  the  leaves 
and,  therefore,  buds.  These  relations  are  seen  best  in  growing  tips  of  stems 
of  not  too  slow  growth,  or  in  seedlings,  just  above  the  levels  at  which  the 
pith-canals  first  come  in.  It  is  also  evident  from  the  positions  taken  by 
the  solitary  canals  which  appear  in  the  epicotyl  before  the  full  complement 
is  established. 

The  very  frequent  anastomosis  and  divarication,  coupled  with  the 
transverse  expansion  of  the  canals,  give  rise  to  a  great  many  columnar, 
trichome-like  structures,  already  alluded  to.  They  lie  approximately 
in  radial  planes,  and  can  be  explained  only  as  imperf orate  longitudinal 
septae  (plate  32,  figs,  i,  6). 

In  older  stems  the  breaking  down  of  the  pith  results  in  the  opening  of 
the  resin-canals,  except  when  plugged  by  pseudotyloses.  There  results  a 
downward  filtration  of  resin  which  finds  its  way  into  the  central  zones  of 
the  old  wood.  This  often  becomes  richly  impregnated  with  resin,  though 
primarily  it  contains  none  at  all.  In  this  way  the  resin-content  of  old 
wood,  shown  by  chemical  methods,  is  to  be  accounted  for  (Lloyd,  1909). 


1  In  the  pith  of  Cynara   carduncula  (Col,   1903)   5  to   10  canals   occur;    in 
Parthenium  hysterophorus  I  count  about  20;   in  P.  lyratum  about  12. 


The  Resin-Canals  in  the  Guayule.  171 

THE  CANALS  IN  THE  LEAF. 

Since  the  canals  in  the  leaves  are  related  only  to  the  primary  cortical 
system,  this  relation  will  now  be  taken  up. 

EARLY  FOLIAGE  LEAVES. 

The  above-mentioned  pair  of  primary  cortical  canals  which  enter  the 
petioles  of  the  earlier  leaves  end  blindly  at  different  levels  in  the  petioles 
and  in  the  leaf-blade1  (plate  38,  figs.  3  to  9).  The  marginal  leaf-traces 
enter  the  petiole  unaccompanied  by  canals,  but  arise  de  novo  in  the  petiole 
dorsal  to  the  lateral  traces.  These  they  follow  into  the  leaf -blade,  and 
branch,  constituting  a  latero-dorsal  system.  The  dorsal  system  may  be 
entirely  absent  from  the  blade  (plate  38,  fig.  5).  There  is  also  a  ventral 
system  composed  of  three  canals,  one  opposite  each  of  three  prominent 
bundles,  namely,  the  median  and  two  lateral.  These  arise  independently 
and  de  novo,  that  opposite  the  median  trace  in  the  petiole,  and  those  oppo- 
site the  lateral  ones,  in  the  blade.  They  originate  analogously  with  the 
pith-canals,  independently  of  the  endodermis2  (plate  37,  figs.  6,  7). 

THE  LATER  LEAVES. 

The  later  leaves,  in  which  their  definitive  character  is  assumed,  re- 
ceive usually  three  to  five  (occasionally  six  or  seven)  cortical  canals  from 
the  stem,  one  with  the  median  and  two  with  each  of  the  stronger  lateral 
traces  (plate  38,  figs.  2,  10  to  18).  These  canals,  which  enter  the  blade, 
follow  the  traces  which  constitute  its  prominent  veins.  The  lateral 
canals  may  branch,  usually  not  more  than  once.  Thus  the  dorsal  system 
of  canals  has,  at  most,  usually  not  more  than  five  ducts.  The  median  canal 
follows  the  midrib  to  the  apex  of  the  leaf.  The  lateral  ends  some  distance 
from  the  apex.  The  ventral  system  arises  de  novo  in  the  petiole  as  three 
to  five  independent  ducts  (plate  38,  figs.  10  to  13),  the  median  arising  first. 
The  lateral  canals  follow  the  main  limbs  of  the  lateral  traces  and  give  off 
branches  to  veins  of  a  higher  order,  until,  in  a  transverse  section,  there 
may  be  five  or  more  on  each  half  of  the  blade.  It  is  thus  seen  that  the 
ventral  system  is  peculiar  to  the  leaf  and  is  more  extensive  than  the  dorsal 
system.  The  canals  anastomose  in  the  upper  part  of  the  blade  and  follow 
the  veins. 

PRIMARY  CANALS  IN  BRANCHES. 

The  primary  system  of  cortical  canals  in  a  branch  is  derived  from  two 
canals  on  either  side  of  the  appropriate  leaf-trace.  At  the  level  at  which  the 
bud  appears,  the  adjacent  canals  in  the  chief  stem  enlarge  radially  and  send 

1  The  behavior  described  is  not  invariable.     One  case  was  found  in  which 
only  one  branch  of  the  canal  anastomosis  entered  the  first  leaf,  while  the  second 
leaf  was  normal,  having  two  canals.     The  third  foliage  leaf  in  this  plant  also  had 
but  one  canal.    One  instance  of  a  leaf  at  about  the  twentieth  node  had  two  canals. 
This  condition  offers  an  analogy  to  that  in  the  cotyledons,  which  may  be  held, 
though  only  tentatively,  as  speaking  for  the  more  primitive  character  of  the 
double  arrangement. 

2  It  is  worth  noting  here  that  there  is  a  single  ventral  canal  opposite  the  mid- 
vein  in  the  cotyledon  of  the  common  sunflower,  Helianthus  annuus. 


172  Guayule. 

off  a  number  of  branches  which  distribute  themselves  in  the  cortex  of  the 
bud.  As  already  said,  generally  a  single  branch  from  the  pith-canal  oppo- 
site the  bud  enters  and  branches  to  produce  the  complement  of  canals 
(plate  39,  figs,  i  and  7). 

SECONDARY  CANALS  IN  ROOT,  HYPOCOTYL,  AND  STEM. 

These  arise,  as  described  by  Ross,  from  special  leptome  parenchyma ' 
derived  directly  from  the  cambium,  and  quite  in  the  same  way  in  all  parts 
of  the  plant.  They  are  at  first  flattened  radially,  opening  out  later  to  be- 
come rounded  or  even  circular  in  transverse  section,  and  finally  becoming 
again  flattened  and  secondarily  distended,  in  company  with  the  growing 
(secondary)  cortex  (plate  22,  fig.  13).  These  canals  constitute  concentric 
branching  and  anastomosing  systems,  each  succeeding 'zone  being  a  sys- 
tem separate  from  all  the  others.  Their  appearance  in  tangential  sections 
(plate  39,  fig.  8)  recalls  the  figure  published  by  Tschirsch  (1906,  p.  1193) 
of  the  canals  in  wound-tissue  in  Larix. 

CANALS  IN  THE  PEDUNCLE. 

It  has  already  been  pointed  out  that  the  inflorescence  is  terminal ; 
the  peduncle  is  therefore  the  morphological  chief  shoot.  I  have  shown 
that  when  an  axillary  bud  develops  it  usually  receives  one  canal  from  the 
pith  (plate  39,  fig.  i).  The  last  bud  formed  on  the  chief  shoot  which  ends 
in  a  peduncle,  however,  receives  all  of  the  canals  from  the  pith,  these  being 
diverted  en  masse.  The  peduncle,  therefore,  contains  no  medullary  canals 
(plate  39,  figs.  2,  3).  Primary  cortical  canals  alone  occur,  there  being  but 
very  little  secondary  thickening. 

Exceedingly  interesting  relations  in  this  regard  are  displayed  by 
rapidly  grown  plants  (plate  39,  figs.  4  to  7).  In  another  chapter  two  types 
of  guayule  have  been  described,  in  one  of  which  the  sharp  delimitation 
between  peduncle  and  foliage  stem  is  not  present.  When  guayule  is  irri- 
gated there  frequently  results,  associated  with  rapid  growth,  a  tendency 
of  the  relatively  chief  shoot  to  run  out  into  inflorescence,2  when  otherwise 
there  would  be  a  sharp  transition  from  stem  to  peduncle,  and  the  upper 
axillary  bud  would  develop  strongly.  When  the  morphological  transition 
is  gradual,  there  is  also  a  correlated  anatomical  transition,  which  the  long 
internodes  make  it  possible  to  analyze.  In  a  specimen  examined,  as  in 
the  normal  condition,  the  peduncle  has  no  pith-canals,  but  the  first  inter- 
node  below  this  has,  instead  of  five,  only  two,  which  pass  into  the  upper- 
most axillary  bud. 

The  sector  of  the  stem  under  the  peduncle  contains  much  more  stere- 
ome,  and  the  two  canals  are  confined  to  the  sector  beneath  the  axillary 
bud,  while  from  the  basal  part  of  the  internode  they  are  absent!  Their 
orientation  above  is  such  as  to  bring  them  opposite  the  first  and  second 
leaves  of  the  axillary  bud;  they  are,  therefore,  the  canals  which  give 
branches  to  the  first  two  axillary  buds  of  the  branch. 

The  axillary  bud  of  the  second  node  below  the  peduncle  receives  from 
the  stem  one  canal  onlv  of  four  which  are  to  be  found  in  the  internode  be- 


1  Secondary  leptome-canals  have  been  described  in  Centrophyllum  lanatunt 
(Col,  U.). 

2  Simulating  the  normal  shoot  in  P.  incanum  (mariola). 


1.  Rubber  in  canal  cells,  nearby  cortex  and  inner 

ray  cells.     Root  1 .2  mm.  diam. 

2.  Older  root.     More  rubber  in  rays. 

3.  Root  2  mm.  diam. 

4.  Parenchyma  ray  from  fig.  2. 

5.  Upper  part  of  hypocotyl,  same  age  as  fig.  1 . 


6.  Longitudinal  section  through  old  wood. 

7.  Longitudinal    section  through    mature  leptome 

parenchyma,  with  a  few  parenchyma  ray  cells. 
6.  Leptome;  elongated  elements. 
9.  Companion  cells  and  sieve  tubes.     No  rubber 

in  younger  leptome  on  the  left. 


2.  Cortex,  stem  of  field  plant  with  maximum  rubber  content. 

3.  Cortex  of  a  20-year-old  stem. 

4.  Root;  rapidly  grown  seedling,  two  months  old.     Rubber  in  granules. 

5.  Rubber  in  process  of  accumulation  in  an  irrigated  plant. 

6.  Primary  resin  canal,  root  1  mm.  diam. 


The  Resin -Canals  in  the  Guayule.  173 

low  the  second  node.  The  other  three  end  blindly  before  they  reach  the 
node,  so  that  the  following  internode  has  none,  as  above  said.  It  is  evi- 
dent that  we  find  here  a  sort  of  morphological  indecision,  as  if  the  stem 
were  trying  to  retain  its  stem  character,  and  still  being  gradually  over- 
come by  the  tendency  toward  changing  into  a  peduncle.  The  same  prepa- 
rations show  also  the  formation  of  chlorenchyma  strips  in  the  cortex  of 
the  peduncle  sector,  nearly  down  to  the  base  of  the  first  internode  below 
the  peduncle. 

The  axillary  bud  of  the  third  node  below  the  peduncle  receives  a 
single  branch  from  one  of  five  canals,  the  normal  number,  present  in  the 
internode  below.  Here,  therefore,  the  complete  stem  structure  is  first 
met  in  our  descent  from  the  peduncle.  It  would  be  interesting  to  specu- 
late on  the  internal  causes  which  result  in  diverting  the  canals,  en  masse, 
from  the  chief  shoot  into  an  axillary  bud. 

THE  CANALS  IN  RETONOS. 

New  shoots  which  take  their  origin  from  roots  have  this  peculiarity 
in  common  with  the  epicotyl,  that  they  do  not  possess  medullary  canals 
till  several  internodes  have  developed.  They  are  further  peculiar  in  lack- 
ing primary  cortical  canals  near  their  bases.  A  retono  23  mm.  long  was 
examined  and  measured.  A  section  near  the  root  at  the  level  of  emerg- 
ence showed  neither  pith  nor  cortical  canals .  At  5  mm .  above  it  five  cortical 
canals  were  found.  At  10  mm.  there  were  three  medullary  canals,  and  at 
1 5  mm.  five  of  these,  so  that  at  the  level  of  1 5  mm.  the  definitive  structure 
had  been  attained. 

In  another  specimen  25  mm.  long,  collected  September  8,  1908,  only 
one  medullary  was  found  at  the  level  of  18  mm.,  and  four  at  21  mm.  In 
still  another,  one  canal  was  found  at  15  mm. 

An  examination  of  a  full  series  of  transverse  sections  through  suc- 
ceeding nodes  and  internodes  discovers  an  important  relation  of  the  pith- 
canals  to  branches,  in  general  harmony  with  the  facts  cited  immediately 
above.  The  material  thus  studied  was  a  retono  several  centimeters  long 
which  developed  in  1908.  The  first  leaf  and  its  axillary  bud  were  devel- 
oped at  the  height  of  20  mm.  The  internode  between  the  mother-root  and 
this  node  had  no  pith-canal.  At  the  first  node  a  single  canal  appeared  just 
below  the  level  of  the  bud,  and  entered  this.  The  succeeding  two  inter- 
nodes (second  and  third)  were  also  devoid  of  canals,  though  at  each  of  the 
corresponding  nodes  a  single  canal  originated  in  the  pith  and  passed  into 
the  axillary  bud.  At  the  third  node,  however,  the  canal  branched,  one 
limb  passing  up  into  the  fourth  internode,  in  the  upper  part  of  which  two 
other  canals  appeared.  One  of  these  three  sent  a  branch  to  the  bud  of  the 
fourth  node,  and  one  ended  blindly,  leaving  two  passing  into  the  fifth 
internode.  At  the  fifth  node  one  of  these  sent  two  branches  into  the  bud, 
two  canals  passing  into  the  sixth  internode.  At  the  sixth  node  both  of 
these  branched,  one  branch  going  into  the  bud  and  three  upward  into  the 
seventh  internode.  At  the  seventh  node  all  three  branched,  one  of  these 
going  into  the  bud,  leaving  the  full  complement  of  five  canals  for  the  suc- 
ceeding internode,  the  eighth.  The  youngest  canal  always  stands  opposite 
the  youngest  bud. 


174  Guayule. 


THE  CONTENTS  OF  THE  CANALS  ;   THEIR  ORIGIN. 

The  very  small  size  of  the  primary  canals  in  the  root  and  hypocotyl 
makes  it  very  difficult  to  determine  the  nature  of  their  contents.  The 
canals  elsewhere  are  known  to  contain  resin  which,  upon  wounding,  exudes 
as  tears,  which  fall  to  the  ground  and  harden  slowly  as  pale  yellow,  limpid 
masses.  The  origin  of  this  secretion  is  of  special  interest  here.  There  is 
no  doubt  that  the  resin  is  confined  to  the  canals,  and  there  is  no  evidence 
that  the  resin  occurs  in  the  protoplasm  of  the  wall-cells  of  the  canal, 
which  have  been  spoken  of  as  secretory.  Treatment  with  alcohol  or  with 
acetone  leaves  the  cell-contents  quite  unchanged  to  all  appearance,  though 
subsequent  staining  with  alkanet  discloses,  when  this  is  originally  the  case, 
a  substance  which  may  be  dissolved  out  by  means  of  xylol  or  other  appro- 
priate solvent,  namely,  rubber.  My  own  observations,  therefore,  give 
support  to  the  general  view,  advanced  by  Tschirch,  that  the  resin  is  to 
be  accounted  for  by  chemical  activity  in  the  outer  part  of  the  cell-walls 
facing  the  meatus.  It  is  not  a  direct  result  of  protoplasmic  activity,  but  of 
enzymatic  activity  in  the  cell -wall  itself.1  It  is  worthy  of  remark  that  the 
wall  (secretory)  cells  of  the  resin- canals  have  the  two-fold  function  of  secret- 
ing rubber  (in  common  with  the  ground-tissue)  within  the  protoplasm  and 
resin  without. 

I  have,  however,  attained  no  success  in  demonstrating  a  mucilaginous 
or  gummy  lining  to  the  meatus,  such  as  is  described  by  Tschirch  (1906, 
p.  1 1 19)  in  many  plants,  to  which  he  ascribes  the  origin  of  resin  formation. 
But  Tschirch  himself  confesses  to  a  similar  difficulty  in  studying,  among 
others,  the  Compositae. 

The  distribution  of  starch  in  the  cortex  and  its  apparent  connection 
with  the  secretion  of  resin  have  been  elsewhere  noted.  The  presence  of 
tannin  in  the  conjunctiva  of  the  young  stem,  especially  associated  with  the 
chloroplasts,  is  to  be  noted,  and  recalls  Tschirch 's  hypothesis  of  the  origin 
of  resin  from  tannin.  The  number  of  Compositae  which  contain  tannin 
is  small,  relatively  to  the  size  of  the  group,  judging  from  the  list  given 
by  Dekker  (1906). 

THE  ROLE  OF  RESIN. 

It  has  often  been  pointed  out2  that  resins  and  ethereal  oils  stand  in 
relation  to  climatic  conditions ,  especially  those  of  the  desert .  The  frequent 
occurrence  of  resin  in  desert  plants  is  a  matter  of  general  observation,  but 
its  function  is  still  a  matter  of  speculation.  Tschirch  rightly  lays  stress 
upon  the  occurrence  of  secretion-containing  structures  near  the  apex  of 
the  young  parts  as  of  significance,  and  this  has  been  pointed  out  for 
the  guayule.  The  evidence  regarding  the  relation  of  resin  to  rubber 
leads  us  nowhere,  and  no  evidence  is  yet  forthcoming  as  to  the  real  role 
of  resin. 

1  Tschirch,  A.    Die  Chemie  und  Biologic  der  pflanzlichen  Sekrete.    Leipzig,  1908. 
1  e.g.,  Tschirch,  1908,  pp.  8-9. 


The  Resin-Canals  in  the  Guayule.  175 

RESIN-CONTENT  OF  GUAYULE  BY  ANALYSIS. 

The  percentage  of  resin  in  the  branches  and  twigs  of  field  plants, 
according  to  figures  obtained  by  Whittelsey  (in  manuscript) ,  is  between 
about  10  per  cent  for  the  smaller  and  about  17  per  cent  for  the  larger 
branches.  The  amount  probably  varies  according  to  the  structure,  and 
this  with  the  rate  of  growth  of  the  parts.  For  irrigated  plants  the  follow- 
ing figures  were  obtained.  The  material  was  the  same  as  that  referred  to 
in  table  53. 

TABLE  51. 

Percentage 
Parts.  of  resin. 

I.  Stump 2.46 

Ila.  Wood  of  1907  growth i  .36 

116.  Cortex  of  this , 4 . 06 

III.  Growths  of  1908  intact 7 . 56 

IV.  New  growth  of  1909  with  leaves 2 . 70 

V.  Roots 10.80 

Aside  from  possible  errors,  it  seems  that,  bulk  for  bulk,  the  irrigated 
plant  contains  less  resin  than  the  field  plant.  This  is  due  in  part  to  the 
larger  relative  volume  of  the  wood  cylinder.  The  reduction  of  the  amount 
in  older  growths  is  due  also  in  part  to  the  radial  compression  of  the  resin- 
canals  in  irrigated  plants,  whereby  their  capacity  is  much  reduced.  The 
force  of  this  explanation  of  the  figures  appears  when  we  compare  the  per- 
centage of  resin  in  III  above.  When  we  introduce  the  rate  of  growth  as 
a  factor  we  must  conclude  that  the  total  secretive  activity  is  not  reduced 
under  irrigation,  nor  is  the  secretive  activity  of  the  resin-secreting  cells 
lowered.  The  result,  however,  is  had  that  in  a  given  volume  of  cortex 
there  is  less  resin  in  irrigated  plants.  In  the  pith,  however,  this  does  not 
hold,  since  the  relative  volume  of  the  resin-canals  under  irrigation  is  as 
great  or  greater  than  in  field  plants.  The  reduced  amount  of  resin  of  the 
cortex ,  volume  for  volume ,  appears ,  therefore ,  to  be  a  secondary  matter  only , 
and  bears,  so  far  as  we  can  see,  no  explanation  in  terms  of  adaptation. 


CHAPTER  VII. 
THE  ORIGIN  AND  OCCURRENCE  OF  RUBBER.1 

Well-nigh  nothing  is  known  about  the  cytology  of  rubber-secreting 
cells.  The  great  initial  difficulties  in  the  investigation  have  arisen  from 
the  fact  that  in  most  rubber-producing  plants  this  material  occurs  in 
latex.  In  the  guayule,  as  in  a  few  other  known  plants,  the  rubber  is  laid 
down  within  certain  cells, in  a  manner  analogous  to  theformation  of  starch. 
Although  the  study  of  the  early  cytological  activities  which  lead  to  the 
accumulation  of  rubber  still  presents  great  difficulties,  since  some  of  the 
agents  used  dissolve  out  the  rubber,  nevertheless  it  has  been  possible  to 
determine  the  relation  of  growth  and  of  some  of  the  more  important  ex- 
ternal conditions  to  rubber  secretion.  These  results  are  important  eco- 
nomically, since  we  are  able  to  determine  the  time  at  which  the  maximum, 
or  near  the  maximum,  amount  of  rubber  occurs,  and  during  what  period 
rubber  is  absent  from  the  new  tissues,  and  thus  establish  rules  of  pro- 
cedure in  the  harvesting  of  the  shrub. 

METHODS. 

The  solubility  of  rubber  in  xylol  and  the  like  prevents  the  use  of 
paraffin.  The  preparations  must  therefore  be  studied  in  such  a  fashion 
that  the  rubber  is  intact.  When  present  in  large  quantities  it  is  easily 
recognized,  after  one  has  become  acquainted  with  its  appearance.2  When 
in  small  quantities,  however,  it  may  easily  be  mistaken  for  droplets  of  oil 
or  resin,  or  for  protoplasmic  or  other  granulations,  and  inasmuch  as  oils 
and  resins  as  well  as  rubber  are  stained  by  alkanet,  these  substances,  if 
present,  must  be  removed  by  suitable  solvents  which  will  leave  the  rubber 
unaffected.  For  this  purpose  I  have  treated  sections  with  high-grade  and 
absolute  alcohols,  acetone,  and  potassium  hydrate,  applying  alkanet  both 
before  and  after.  There  remains  the  possibility  that  the  substances  which 
remain  and  which  react  to  alkanet  are  not  always  rubber  in  its  final  form, 
but  there  can  be  little  doubt  that  the  materials  which  are  referred  to  below 
are  either  rubber  or  are  substances  in  the  course  of  change  into  rubber. 
The  evidence  seems  to  indicate,  however,  that  it  is  rubber  which  we  are 
dealing  with. 

In  seeking  to  determine  with  accuracy  the  facts  of  the  distribution 
of  rubber  in  the  tissues,  the  accident  of  displacement  of  rubber  in  the  act 
of  sectioning  must  be  properly  guarded  against.  When  rubber  is  present, 

1  The  substance  of  this  chapter  was  presented  in  a  paper  entitled  "The 
responses  of  the  guayule,  Parthenium  argentatum  Gray,  to  irrigation,"  before  the 
Botanical  Society  of  America,  Boston,  December  1909. 

*  When  in  readily  appreciable  quantities,  resin  and  rubber  in  the  guayule  may 
readily  be  distinguished  by;  alkanet.    Resin  takes  on  a  brilliant  scarlet,  while  rubber 
has  a  purplish  tinge,  and  is,  to  the  naked  eye,  blood-red. 
176 


1.  Apex  of  terminal  twig  of    1908,  field 

plant,  July  22. 

2.  Near  base  of  same. 

3.  Pseudo-tylose  with  rubber  in  the  cells. 

4.  Leptome,  field  plant. 


5.  Pith  of  a  field  stem,  10  mm.  diam. 

6.  An  old  leaf  trace. 

7.  Outer  cortex  of  a  field  stem. 

8.  Outer  edge  of  cortex  and  inner  zone  of 

cork  derived  from  collenchyma. 


The  Origin  and  Occurrence  of  Rubber.  177 

the  contents  of  the  cells  ruptured  during  the  sweep  of  the  knife  agglomer- 
ate and  stick  in  irregular  masses  to  the  section.  It  is  also  to  be  suspected 
that  particles  of  rubber  displaced  from  one  cell  may  remain  attached  to 
other  cells  in  such  a  manner  as  to  simulate  an  original  position  in  them. 
This  danger  is  greater  where  the  particles  are  small,  since  with  smaller 
size  the  chance  against  agglomeration  is  greater.  With  some  experience, 
however,  this  general  difficulty  is  reduced,  so  that,  with  proper  observa- 
tion, mistakes  are  easily  avoided.  The  guiding  principle  of  observation  is 
simply  to  confine  study  to  uninjured  cells. 

GENERAL  DISTRIBUTION  OF  RUBBER  IN  THE  PLANT. 

It  has  been  known  for  some  years l  that  the  rubber  in  guayule  occurs 
in  the  parenchyma  "  cells  "  of  the  stem  and  root ;  that  is,  in  the  pith,  paren- 
chyma rays,  and  cortex  (the  conjuctiva,  in  a  word).  These  facts,  though 
known  to  a  few,  were  first  clearly  stated  by  Ross  in  1 908,  according  to  whom 
rubber  occurs  in  almost  all  the  cells  of  the  ground-tissue  in  root  and  stem ; 
that  is,  in  those  of  the  pith,  parenchyma-rays,  primary  cortex,  and  also  in 
the  wood-parenchyma.  The  leaves,  he  adds,  contain  little  or  none.  While 
I  have  been  able  to  confirm  these  conclusions  in  general,  several  additional 
details  have  come  to  light. 

Rubber  occurs  invariably  in  all  the  cells  of  the  resin-canals  (plate  41, 
figs.  4-6) .  While  I  am  unable  to  state  positively  that  it  occurs  here  earlier 
than  elsewhere,  it  certainly  is  secreted  most  rapidly.  There  is,  however, 
evidence  that  the  former  statement  is  true. 

In  the  primary  hadrome  parenchyma  rubber  does  not  occur  early. 
In  the  preparations  from  which  the  photographs  on  plate  42  (figs,  i  and  2) 
were  taken,  there  was  no  trace  of  rubber.  There  can  be  no  doubt,  how- 
ever, that  rubber  is  secreted  by  some  or  all  of  these  cells  later  on,  as  they 
are  replete  after  some  secondary  thickening  has  occurred  in  the  leaf -trace, 
which,  of  course,  suffers  no  secondary  change2  (plate  42,  fig.  6).  The  cells 
of  uniseriate  parenchyma  rays,  in  consonance  with  other  parenchyma-ray 
cells,  contain  rubber. 

In  the  primary  leptome,  under  the  conditions  noted  for  the  primary 
hadrome  in  the  preceding  paragraph,  rubber  occurs  at  least  in  the  fiber- 
cells  and  in  the  parenchyma  (plate  42,  fig.  6).  This  takes  place  after  the 
abscission  of  the  corresponding  leaf.  In  secondary  leptome  I  have  seen 
small  amounts  in  mature  fiber-cells,  just  before  sclerosis  sets  in.  Occur- 
rence of  rubber  in  these  elements  appears,  therefore,  to  be  a  function  of 
age.  It  is  secreted  normally  in  leptome-parenchyma,  and  at  the  same 
time  is  in  adjacent  parenchyma-ray  cells  (plate  42,  fig.  4). 

In  the  secondary  leptome  rubber  may  also  be  seen  in  all  the  elements 
of  the  sieve-tissue  (plate  40,  figs.  8,  9).  It  is  true  that  the  very  narrow 

1  Fron  and  Franjois,  1901. 

2  The  parenchyma  of  the  secondary  hadrome  is  rather  scanty  and  of  small 
elements.     They  do  not  secrete  rubber  as  early  as  the  medullary-ray  cells  in  the 
same  zone,  but  ultimately  do  so.     The  rubber  may  best  be  seen  in  longitudinal 
sections,  treated  with  boiling  10  per  cent  caustic  potash  and  stained  with  alkanet. 
The  rubber  then  appears  as  small  series  of  globules.  The  wood  may  also  be  macerated 
by  means  of  Schultz's  medium,  and  later  stained. 

12 


178  Guayule. 

elements  contain  very  little,  but  this  amount  may  be  clearly  demonstrated 
in  longitudinal  sections  treated  as  above  described. 

In  the  cork-cells  rubber  occurs  in  a  secondary  condition  as  small 
droplets,  derived  by  the  breaking  up  (possibly  an  emulsification)  of  the 
compact  masses  in  the  outer  cortex.  These  droplets  are  larger  in  cork-cells 
on  either  side  of  collenchymatic  zones  which  are  remnants  of  the  periclinal 
walls  of  collenchyma  (plate  42,  fig.  8). 

Rubber  is  secreted  in  the  parenchyma  of  the  pseudotyloses  (plate  42, 
fig.  3),  quite  as  in  the  adjacent  cells. 

In  the  leaf  the  amount  of  rubber,  though  always  small,  is  in  propor- 
tion to  its  age.  In  the  oldest  leaf  I  have  observed,  rubber  occurs  in  drop- 
lets in  the  outermost  palisade-cells  of  both  surfaces,  and  less  conspicuously 
in  the  subjacent,  but  usually  in  no  other  chlorenchyma  cells  (plate  43,  fig. 
5).  It  may  occur  only  in  the  ventral  palisade  in  younger  leaves.  In  very 
minute  droplets  it  is  to  be  found  also  in  the  collenchyma  and  endodermis 
of  the  midvein  and  in  nearly  all  of  the  non-chlorophyllous  cells  in  the 
region  about  it,  and  in  the  leptome,  both  in  the  companion  and  sieve  cells. 
Curiously  enough,  it  is  not  to  be  found  in  the  secreting-cells  of  the  resin- 
canals,  though,  on  the  other  hand,  it  is  in  small  but  conspicuous  quantities 
in  the  subjacent  cells.  A  minute  amount  occurs  in  the  epidermis,  espe- 
cially near  the  midvein,  and  in  the  non-chlorophyllous  cells  near  the  smaller 
veins.  The  maximum  quantity,  negligible  from  the  economic  point  of 
view,  occurs  in  the  oldest  leaves  which  have  passed  through  a  drought 
period.  The  material  which  gave  these  results  was  collected  in  the  spring 
of  1909  before  the  summer  rain  of  that  year. 

In  material  collected  from  irrigated  plants  at  Cedros  in  April  1909 
rubber  may  be  detected  in  exceedingly  minute  quantities  in  the  basal  part 
of  the  leaf  only.  A  single  minute  droplet — not  more  than  one-fourth  the 
diameter  of  those  seen  in  the  field  plant — may  be  seen  in  each  outermost 
palisade-cell  of  the  upper  (ventral)  surface.  They  are  a  trifle  larger  near 
the  midvein.  In  the  non-chlorophyllous  tissue  near  this  the  rubber  may 
also  be  detected  in  still  more  minute  quantities. 

Since  within  the  periphery  of  the  wood  cylinder  only  the  conjunctiva 
and  a  small  amount  of  wood-parenchyma  contain  rubber,  and  since  in  older 
wood  the  medullary  rays  (in  part)  and  the  pith  and  its  canals  are  dead 
and  disintegrated,  the  wood  cylinder  contains  less  rubber  than  the  cortical 
tissues,  but  it  is  also  less  resinous  in  its  primary  condition.  Recent  work 
by  Whittelsey  (1909),  however,  indicates  that  in  stems  of  an  advanced 
age,  at  any  rate,  the  amount  of  true  rubber  is  practically  reduced  to  nil, 
though  in  the  young  twigs  the  proportion  of  rubber  within  the  periphery 
of  the  wood  cylinder  is  large.  We  must  conclude,  therefore,  that  the 
rubber  in  the  older  wood  undergoes  chemical  change,  and  is  broken  down 
into  related  materials.  There  is  no  doubt  that  some  such  change  takes 
place  also  in  the  secondary  cortical  tissues  cut  out  by  the  inner  periderm, 
and  this,  as  I  have  shown,  is  a  considerable  part  of  the  volume  of  the 
"bark"  in  older  stems. 


The  Origin  and  Occurrence  of  Rubber.  179 

APPEARANCE  OF  RUBBER  IN  RICHLY  LOADED  TISSUES. 

A  section  taken  through  any  young  stem  of  a  field  plant  after  some 
period  of  drought  will  give  a  typical  appearance  (plate  42,  fig.  7).  All  the 
cells  of  the  conjunctiva  appear  to  be  filled  with  a  gray  substance.  A  good 
deal  of  it  will  have  been  swept  out  of  ruptured  cells  by  the  knife-edge  and 
agglomerated,  the  resulting  masses  having  irregularly  rounded  outlines, 
with  strands  stretching  here  and  there,  still  attached  to  the  tissue.  These 
masses,  seen  obscuring  the  pith  to  some  extent  in  plate  43,  fig.  2,  and  the 
dense  cell-contents  stain  deeply  with  alkanet,  the  stain  being  more  bril- 
liant if  the  sections  have  been  previously  boiled  in  a  10  per  cent  solution 
of  caustic  potash.  If  the  sections  have  not  been  acted  on  by  alcohol  or 
potash  drops  of  yellow  resin  will  be  seen  in  the  resin-canals,  but  nowhere 
else,  except  accidentally. 

Closer  examination  of  the  rubber  within  the  cells  shows  that  the  mass 
is  not  homogeneous,  and  does  not  entirely  fill  the  cavity.  It  may  form  a 
heavy  layer  about  the  wall,  leaving  a  more  or  less  irregular  space  within, 
or,  if  apparently  filling  the  entire  cell,  it  will  contain  numerous  spherical 
spaces  (plate  41 ,  figs.  1,2).  Sections  which  have  lain  in  glycerin  may  show 
the  masses  tobe  contracted,  owing  to  a  plasmolytic  action  upon  them,  from 
which  it  is  to  be  inferred  that  they  have  a  considerable  water-content, 
held  within  the  vacuoles,  in  part  at  least  (plate  41 ,  fig.  3) .  The  rubber  may 
also  accumulate  as  a  round  drop  within  the  vacuole  of  the  cell  (plate  42, 
fig.  7),  its  size  depending  upon  the  age  of  the  cell.  Plasmolysis  shows 
further  that  all  the  parenchyma  cells  are  not  equally  densely  filled,  though 
of  the  same  age.  This  is  often  conspicuously  the  case  when  the  cells  of 
the  cortex  and  those  of  the  adjacent  parenchyma  rays  are  compared.  In 
the  cortical  cells  the  rubber  forms  a  dense  rounded  drop  (plate  42,  fig.  7), 
while  the  cytoplasm  may  be  seen  between  it  and  the  cell  wall.  In  paren- 
chyma-ray cells  the  rubber  mass  is  frequently  irregular,  full  of  irregular 
vacuoles,  and  the  cytoplasm  appears  usually  to  have  shrunk  away  with 
it.  In  the  parenchyma-ray  cells  in  some  preparations  it  is  quite  as  regular 
as  in  the  adjacent  cortical  cells,  but  appears  to  be  more  dense,  owing  to  a 
very  much  larger  number  of  minute  spaces.  This  difference,  in  one  form  or 
the  other,  is  quite  constant,  and  seems  to  indicate  that  the  rubber-content 
of  the  cortical  cells  is  higher  than  that  of  the  adjacent  parenchyma-ray 
cells. 

Cortex  which  has  been  cut  out  en  masse  by  inner  periderm  also  con- 
tains rubber.  In  the  cells  of  this  tissue  it  has  a  still  different  appearance, 
being  segregated  into  droplets  of  various  sizes,  in  a  fashion  to  suggest  the 
analogous  appearance  of  dead  protoplasm.  In  newly  formed  cork-cells 
proper,  just  outside  of  the  periderm,  a  different  behavior  is  seen. 

BEHAVIOR  OF  PERIDERMAL  DIVISIONS  TOWARD  RUBBER. 

Since  the  secondary  cortical  cells  in  field  plants  contain  a  large 
amount  of  rubber  in  the  condition  described,  the  fact  that  the  cork-cells 
immediately  outside  of  the  actively  dividing  suberogenous  cells  may  contain 
no  rubber  at  all,  or  only  occasionally  a  small  amount,  calls  for  explanation. 
The  suberized  walls  of  the  cork  take  up  alkanet  readily,  so  that,  after 


180  Guayule. 

treatment  with  that  reagent,  the  contrast  between  the  rubber-containing 
cells  of  the  cortex  and  the  empty  nearby  cork-cells  is  very  clear  and  striking. 
Inasmuch  as  the  peridermal  divisions,  though  several  times  repeated  in  the 
same  mother-cell,  finally  involve  a  considerable  depth  of  tissue,  and  as 
the  rubber  can  not  travel  from  cell  to  cell  as  such,  we  must  conclude  either 
that  the  rubber  is  translocated,  which  is  unlikely,  or  that  it  disintegrates. 
In  support  of  the  latter  conclusion  we  note  the  following  ocular  evidence: 

1.  When  the  first  cork-cambium  division  takes  place  the  partition 
passes  through  the  rubber-content,  whereby  the  two  daughter-cells  each 
receive  a  share  (plate  31,  fig.  14).     From  the  outer  cell,  which  becomes 
suberized,  the  rubber  disappears. 

2.  This  disappearance  is  gradual.     The  rubber  may  first  break  up 
into  droplets,  which  become  fewer  in  number  till,  in  the  second  series 
of  cork-cells,  scarcely  any  evidence  of  its  former  presence  remains,  or  it 
may  become  shrunken  in  appearance.    During  this  time  the  rubber,  if  it 
still  is  such,  reacts  less  characteristically  to  alkanet,  and  takes  on  a  dirty 
bluish  tint.    In  one  young  root,  however,  I  observed  droplets  of  rubber 
giving  the  characteristic  stain,  out  several  cells  distant  in  the  cork.    The 
explanation  may  be  that  after  the  death  of  the  protoplasm  the  oxidizing 
enzymes  present  hasten  the  disintegration.    This  may  be  less  rapid  in  the 
root,  though  it  is  difficult  to  say  why.     The  mere  contact  with  the  air 
would  seem  an  insufficient  explanation,  since  disintegration  of  the  rubber 
in  cortex  cut  out  bodily  by  inner  periderm  is  very  slow. 

THE  DEVELOPMENT  OF  RUBBER  IN  THE  CELL. 

All  that  we  are  able  to  do  microscopically  in  regard  to  the  method  of 
origin  of  rubber  in  the  cell  is  to  detect  its  first  appearance  and  the  subse- 
quent accumulation,  and  we  are  therefore  precisely  in  the  position  of 
the  poet  who  said  of  a  matter  usually  regarded  as  far  removed  from  the 
realm  of  science, 

"Sie  kommt,  und  sie  ist  da." 

We  are  unable  to  say  at  this  point  whether  the  origin  is  associated  with 
special  organs  as  plastids  or  not,  though  my  observations  up  to  the  present 
indicate  that  there  are  no  such  organs. 

The  relation  of  nuclear  activity  in  general  to  secretion  is  well  known. 
The  rubber  in  the  palisade-cells  of  the  leaf  appears  first  in  all  cases  in 
contact  with  the  nuclear  membrane,  and  for  this  reason  does  not  take  the 
form  of  spherical  but  of  concavo-convex  droplets,  seen  in  plate  43,  fig.  5. 
Elsewhere  the  earliest  appearance  is  as  very  minute,  well-nigh  invisible 
droplets  (plate  41,  fig.  4),  scattered  in  the  protoplasm.  They  grow  in 
size  and  increase  in  numbers  until  the  protoplasm  is  loaded  sufficiently 
to  render  it  exceedingly  frothy  in  appearance  (plate  41,  fig.  5).  These 
droplets  may  travel  toward  the  interior  of  the  cell  and  be  extruded  into 
the  vacuole,  where  they  run  together  to  form  a  larger  droplet  or  a  more 
or  less  irregular  mass.  This  is  not  homogeneous,  as  might  be  supposed, 
but  is  vacuolated,  sometimes  so  much  so  that  it  is  quite  alveolar  in  struc- 
ture (plate  41,  fig.  i),  sometimes  less  so,  the  vacuoles  being  widely  scat- 


The  Origin  and  Occurrence  of  Rubber.  181 

tered.  That  these  vacuoles  contain  various  substances  in  solution  in  the 
inclosed  water  can  not  be  doubted,  and  it  seems  likely  that  among  these 
are  enzymes1  which  may  act  upon  the  rubber  after  extraction  by  the 
mechanical  processes  in  vogue.  It  also  seems  likely  that  the  protoplasm 
of  the  cells  becomes  intermingled  with  the  rubber  during  extraction,  ren- 
dering it  more  or  less  albuminous  and  liable  to  give  off  the  products  of  the 
decay. 

CENTERS  OF  SECRETION. 
THE  ROOT. 

With  certain  exceptions,  the  secretion  of  rubber  both  in  the  stem 
and  the  root,  including  the  hypocotyl,  appears  to  proceed  from  definite 
centers.  This  is  exemplified  with  especial  clearness  in  the  root,  where,  in 
the  cortex,  the  secreting-cells  of  the  resin-canals2  are  the  first  to  show  the 
presence  of  granules  of  rubber  (plate  41,  fig.  6).  It  is  argued  that  secre- 
tion actually  begins  earlier  in  these  cells  because  the  surrounding  cortical 
cells,  primary  on  the  outside,  secondary  on  the  inside,  contain,  at  an  early 
stage  of  secretion,  less  and  less  rubber,  as  one  proceeds  farther  from  the 
canals.  The  figures  of  plate  40  illustrate  this  advance  in  secretion,  the 
beginning  of  which  is  seen  in  a  young  stage  in  the  development  of  the 
root  (plate  23,  figs.  3,  7;  plate  40,  fig.  i).  If  the  rate  of  growth  has  not 
been  too  rapid,  so  that  a  part  of  the  primary  cortex  has  had  the  necessary 
time  to  secrete  rubber  before  being  cast  off,  the  activity  of  secretion  is 
seen  to  be  taken  up  successively  by  the  cells  further  removed,  until  the 
whole  tissue  becomes  loaded  (plate  40,  figs.  2,3).  The  greater  amount  of 
rubber,  however,  is  evidently  held  by  the  cells  nearer  the  resin-canals. 
In  the  hypocotyl  the  same  physiological  relations  hold. 

The  secretive  activity  of  the  secondary  cortex  is  taken  up,  aside  from 
those  cells  in  the  neighborbood  of  the  canals,  by  successive  layers  of  cells, 
beginning  on  the  outside.  With  the  appearance  of  the  secondary  resin- 
canals,  however,  a  superior  activity  in  rubber  secretion  in  their  secreting- 
cells  is  to  be  early  noted. 

On  the  other  hand,  simultaneously  with  the  appearance  of  rubber  in 
the  primary  canal-cells,  it  appears  also  in  the  innermost  cells  of  the  paren- 
chyma rays,  the  function  of  secretion  being  taken  up  successively  by  the 
next  outer  cells,  and  so  on.  This  is  apparent  in  the  figures  (plate  40,  figs,  i 
to  4).  If  a  period  of  rapid  growth  follows  one  of  stasis,  the  newly  formed 
parenchyma-ray  tissues  will  show  an  entire  absence  of  rubber  (plate  40, 
ng-  3)-  When  secretion  again  begins,  it  starts  simultaneously  in  the 
outermost  and  innermost  cells  of  the  parenchyma  ray. 

THE  HYPOCOTYL. 

In  the  hypocotyl  a  similar  condition  prevails,  though  here,  as  in  the 
definitive  stem,  there  is  a  pith.  That  is,  the  innermost  parenchyma-ray 
cells  assume  secretive  ability  earlier  than  the  pith-cells,  which  is  not  true 
for  the  definitive  stem  (plate  40,  fig.  5). 

1  The  presence  of  oxidases  in  extracted  rubber,  both  in  latex  rubbers  (Spence, 
1909)  and  in  guayule  rubber,  is  known. 

2  In  view  of  the  emphasis  which  has  been  placed  by  many  writers  on  the  endo- 
dermis  as  seat  of  high  physiological  activity,  the  beginning  ot  the  secretion  of  rub- 
ber in  the  resin-canal  cells,  which  are  constituents  of  the  endodermis,  is  of  very  great 
interest. 


182  Guayule. 

THE  STEM. 

In  the  stem,  the  first  evidences  of  rubber  are  to  be  observed  in  the 
secreting-cells  of  the  cortical  and  medullary  canals  simultaneously.  The 
dark  appearance  of  these  cells  in  figure  i ,  plate  42,  is  due,  in  part,  to  their 
larger  rubber-content,  but  in  part  to  the  denser  protoplasm.  The  condi- 
tion to  be  seen  in  these  cells  is  represented  by  the  camera  drawing  in  plate 
31,  figs.  10  and  12.  The  section  was  taken  toward  the  apex  of  a  newly 
grown  twig  of  a  field  plant,  collected  on  July  22,  1908,  and  was  then  about 
six  weeks  old.  In  all  the  cells  of  the  conjunctiva  very  minute  granules  of 
rubber  could  be  seen,  but  not  more  in  the  cells  near  the  canals  than  else- 
where. In  the  stem,  therefore,  secretion  appears  to  begin  first  simultane- 
ously in  the  canal-cells  of  the  pith  and  cortex,  and  then  in  the  conjunctiva. 
It  is,  however,  quite  readily  determined  that  the  physiological  activity  of 
the  pith  is  greater  than  that  of  the  cortex.  In  fig.  2,  plate  42,  is  shown 
a  section  taken  from  the  twig  just  mentioned,  but  near  the  base  of  the 
new  growth.  One  or  two  peridermal  divisions  have  ensued,  while  other 
secondary  changes  may  be  noted.  The  rubber-content  of  the  pith-cells  is 
obviously  greater  than  that  of  the  cortex  in  this  section.  Further,  I  have 
noted  in  irrigated  plants  that  the  amount  of  rubber  is  greater  in  the  outer 
than  in  the  inner  cortical  cells  (plate  43,  fig.  i).  It  seems,  therefore,  that 
the  deportment  of  both  root  and  stem  is  essentially  the  same  and  that  the 
hypocotyl,  though  possessing  a  pith,  behaves  as  the  root. 

During  secondary  thickening,  as  in  the  root,  the  secondary  cortical 
canals  exhibit  early  activity  in  rubber  secretion,  while  this  is  taken  up  by 
the  oldest  parenchyma-ray  cells  first,  simultaneously,  therefore,  at  the 
inner  and  outer  edges. 

THE  LEAF. 

In  the  leaf  the  earliest  appearance  of  rubber  is  in  the  outer  palisade 
in  the  ventral  moiety.  I  found  rubber  in  these  cells  only  in  old  leaves  of 
irrigated  plants.  The  analogy  with  the  condition  described  for  the  stem, 
in  which  superior  activity  is  shown  by  the  pith,  is  clear.  But  the  failure 
of  the  leaf-canal  cells  to  show  greater  activity  than  the  neighboring  con- 
junctiva detracts  from  the  force  of  the  comparison.  The  leaf  observed 
by  me  to  be  most  richly  supplied  with  rubber  contained  a  single  drop- 
let, with  a  diameter  about  half  the  transverse  diameter  of  the  cells,  in 
each  palisade-cell  toward  the  median  vein.  The  amount  of  the  rubber 
became  less  and  less  toward  the  margin.  This  was  true  also  of  the  outer 
palisade  of  the  dorsal  (lower)  surface,  and  in  a  less  degree  of  the  inner 
palisade. 

Minute  granules  occurred  also  in  all  the  non-chlorophyllous  cells, 
mechanical  and  conjunctive,  forming  the  midrib,  excepting  the  vascular 
and  sieve  elements.  It  would  seem,  therefore,  that,  roughly  speaking,  the 
midvein  is  the  center  of  rubber  secretion,  which  proceeds  through  the 
lamina  toward  the  margins;  further,  that  activity  is  shown  first  by  the 
outer  palisade-cells,  then  by  the  inner,  and  first  by  the  ventral  and  later 
by  the  dorsal.  In  this  regard,  as  already  said,  the  analogy  to  the  stem  is 
clear. 


The  Origin  and  Occurrence  of  Rubber.  1 83 

RATE  OF  RUBBER  SECRETION  RELATIVE  TO  GROWTH. 

The  material  which  I  have  studied  in  order  to  determine  the  relation 
of  growth  to  the  rate  of  rubber  secretion  was  collected  during  and  follow- 
ing the  growing-season  of  1908,  which  began  about  June  i.  Growth  is 
rapid  for  the  first  part  of  the  season,  during  which  several  centimeters  of 
stem-length  are  attained  and  one  to  three  flower-stalks  are  developed.  A 
period  follows  in  which  there  is  little  lengthening,  and  more  or  less  second- 
ary thickening  occurs,  according  to  the  length  of  the  period  during  which 
growth  of  any  kind  may  take  place.  During  the  first  part  there  is  no  evi- 
dence of  secretion  of  rubber  in  the  new  parts ;  during  the  second,  which 
began  in  1908  in  late  July  or  August,  there  is  a  slight  evidence  of  secretive 
activity  as  regards  rubber,  though  the  secretion  of  resin  is  synchronous 
with  growth.  The  relation  may  best  be  expressed  by  saying  that  the  secre- 
tion of  rubber  is  a  secondary  physiological  process,  its  rate  of  appear- 
ance being  inversely  to  the  rate  of  growth.  The  rate  relation  is  brought 
out  best  by  plants  grown  under  experimental  conditions,  in  which  the 
more  rapid  growth  is  accompanied  by  a  less  rapid  secretion  of  rubber.  No 
exact  quantitative  statement  can  be  made,  since  the  conditions  under 
which  experimental  plants  have  been  grown  have  not  been  fully  con- 
trolled. In  studying  material,  I  have  tabulated  numerous  observations 
in  field  and  irrigated  seedlings,  of  various  ages  and  at  different  periods  of 
the  year,  and  compared  the  rubber-content  of  the  cells  in  all  the  tissues 
with  that  in  irrigated  seedlings.  The  same  has  been  done  for  mature  field 
and  irrigated  plants.  For  this  purpose  the  material  which  has  frequently 
been  alluded  to  was  at  hand,  viz,  the  branches  and  stocks  of  irrigated 
plants  at  Cedros  (plate  4,  fig.  B)  and  at  Caopas  (plate  46,  fig.  B),  both 
immediately  at  the  close  of  growth-periods  and  after  a  period  of  drought. 
The  attempt  was  made  to  grade  the  preparations  on  the  rubber-content 
of  the  cells,  and  while  this  method  of  procedure  has  little  to  recommend 
it  for  more  than  approximate  accuracy,  it  enables  us  to  draw  reasonable 
conclusions  as  to  the  rate  of  progress  of  secretion.  My  observations  have 
been  digested  in  the  following  notes,  which  will  serve  to  present  sufficient 
concrete  evidence  to  support  my  conclusion. 

1.  At  the  close  of  the  dry  season  (May  1908)  all  the  cells  of  rubber- 
bearing  tissues  produced  by  growth  during  1907,  both  in  new  shoots  and 
in  new  tissues  in  older  shoots  in  field  plants,  contained  rubber  in  maximum 
quantities  (plate  42,  fig.  7). 

2.  The  same  may  be  said,  generally,  for  the  field  seedlings.    There  is, 
however,  evidence  that  in  the  cells  of  the  pith  near  the  top  of  the  seedling 
the  maximum  content  of  rubber  is  not  reached.    Seedlings  (plate  1 7 ,  fig. 
A)  of  rapid  growth  in  1908  had  not  reached  the  maximum  content  (as 
shown  both  microscopically  and  by  the  analysis  on  p.  187)  in  April  1909. 
In  the  cells  of  the  root  it  was  more  densely  agglomerated  than  in  the  stem. 
Here  the  rubber  had  the  same  appearance  as  in  irrigated  plants.    It  was 
only  partly  agglomerated,  and  only  partially  filled  the  cells.    It  is  quite 
probable  that  this  condition  occurs  occasionally  in  mature  plants  in  drier 
habitats  after  exceptional  rainfall  and  regularly  in  moister  conditions. 

3.  A  medium-sized  twig,  grown  in  1908,  beginning  about  June  i, 
measuring  3.3  mm.  in  diameter  at  the  base  and  1.2  mm.  at  the  tip,  was 


184  Guayule. 

examined  Aug.  14.  At  the  base  the  rubber  in  the  pith  was  finely  granu- 
lar, showing  in  addition  a  tendency  to  agglomeration  (plate  42,  fig.  2) ;  in 
the  extreme  inner  and  outer  cells  of  the  parenchyma  rays  the  rubber  was 
very  finely  granular,  while  in  the  cells  lying  on  either  side  of  the  cambium 
there  was  none  or  extremely  little;  in  the  primary  cortex  it  was  finely 
granular,  but  was  in  somewhat  larger  granules  in  the  secondary  cortex; 
large  granules  occurred  in  the  younger  resin-canal  cells  (in  the  secondary 
cortex)  and  agglomerated  masses  in  the  older  canal  cells  (in  primary 
cortex  and  pith) .  Near  the  apex  of  the  stem  the  rubber  was  found  only 
in  extremely  minute  granules  everywhere  (plate  42,  fig.  i)  excepting  in  the 
resin-canal  cells,  where  they  were  somewhat  larger,  but  still  small  (plate  3 1 , 
figs.  10  to  12). 

4.  A  similar  twig,  examined  Sept.  8,  showed  that  the  condition  seen 
at  the  base  in  the  twig  described  immediately  above  had  advanced  toward 
the  apex  about  one-third  the  length  of  the  twig.    At  the  base  the  rubber 
had  increased  till  it  had  become  coarsely  granular,  except  in  the  paren- 
chyma-ray cells  nearer  the  cambium,  in  which  it  was  still  finely  granular. 
Five  mm.  from  the  apex  there  was  still  scarcely  sufficient  rubber  to  be 
observable,  except  in  the  resin-canal  cells. 

I  was  unable  to  obtain  material  during  the  succeeding  few  months, 
so  was  prevented  from  following  the  march  of  secretion  after  September  8. 
It  is,  however,  clear  that  the  rate  of  secretion  is  so  slow,  as  compared  with 
the  rate  of  growth,  that  for  at  least  four  months  after  the  beginning  of  the 
rainy  season  the  new  parts  contain  only  very  small  quantities  of  rubber. 
From  this  time  on  the  secretion  of  rubber  probably  proceeds  more  rapidly, 
but  it  is  still  to  be  determined  when  the  maximum  is  reached.  This  is  a 
point  of  very  great  importance. 

5.  Turning  to  irrigated  plants,  I  need  cite  the  evidence  from  only 
three  examinations: 

(a)  A  branch  (plate  2 1, figs.  A,  B)  of  a  Cedros plant  (plates 4 and  17,  fig. 
B)  which  began  to  grow  in  1907  and  was  examined  in  August  1908.     In 
examining  the  1907  growth  no  rubber  was  detected  in  the  pith,  probably 
because  the  small  amounts  secreted  in  1907  had  disintegrated;  the  older 
cells  (of  1907)  in  the  parenchyma  rays  contained  rubber  in  fine  granules 
near  the  cortex;  in  the  cortex  and  resin-canal  cells  there  were  coarse 
granules  with  more  or  less  agglomeration.    The  new  tissues  of  1908  con- 
tained only  very  minute  granules.     In  the  1908  growth,  near  the  base,  the 
rubber  was  visible  in  very  fine  granules,  save  in  the  primary  cortex,  where 
there  was  none;  in  the  resin-canal  cells  coarse  granules,  these  still  larger 
in  the  pith-canals;  midway  between  the  base  and  apex  there  were  very 
fine  granules  of  rubber  in  the  pith  and  parenchyma  rays ;  the  resin-canal 
cells  had  coarse  or  agglomerated  granules ;  fine  granules  were  visible  in  the 
secondary  cortex,  but  none  in  the  primary.     Four  centimeters  from  the 
apex,  where  the  stem  was  still  herbaceous,  minute  granules  of  rubber 
had  appeared  only  in  the  pith  and  inner  parenchyma-ray  cells  nearby ; 
it  was  present  in  coarse  granules  in  the  resin-canal  cells  of  the  pith,  and 
in  fine  granules  in  those  of  the  cortex ;  the  cortex  itself  contained  none 
(plate  43.  %•  i). 

(b)  A  branch  from  a  single  Cedros  plant  collected  in  April  1909  (plate 
17,  fig.  B),  after  a  prolonged  drought  extending  with  practically  no  inter- 


The  Origin  and  Occurrence  of  Rubber. 


185 


ruption  from  August  1908.  Rubber  was  found  in  dense  rounded  agglom- 
erations throughout,  but  evidently  not  reaching  a  maximum  content 
(plate  43,  fig.  2). 

(c)  A  branch  from  a  plant  grown  at  Caopas,  from  stocks  transplanted 
by  Don  Teofilo  Delgadillo  about  January  1908  and  taken  in  October  1909. 
These  had  less  irrigation  than  the  above-mentioned  Cedros  plants.  1908 
growth:  the  rubber  was  densely  agglomerated  in  the  whole  of  the  con- 
junctiva (plate  43 ,  figs.  3 , 4) ,  in  amounts  exceeding  that  in  Cedros  material 
(plate  43,  fig.  a);  the  1909  growth  contained  rubber  in  coarse  granules 
more  or  less  agglomerated  throughout. 

6.  Irrigated  seedlings  of  all  ages  up  to  five  months  were  examined. 
Very  young  individuals  were  seen  which  contained  no  rubber  at  all.  A 
five-months-old  seedling  (plate  20,  fig.  B)  contained  rubber  in  coarse 
granules  throughout  the  conjunctiva,  being  in  sufficient  quantity  in  the 
secondary  cortex  to  become  agglomerated. 

The  method  which  was  used  in  obtaining  the  foregoing  data,  despite 
its  limitations,  could  doubtless  be  used  by  the  grower  of  guayule,  enabling 
him  to  follow  the  behavior  of  the  plants  under  his  charge.  The  evaluation 
of  the  evidence  is  somewhat  difficult,  but  it  could  be  mastered,  as  may 
be  seen,  I  think,  on  examining  plates  40  to  43.  The  final  control  must, 
however,  be  had  by  chemical  analysis.  Tables  52  to  54,  which  follow, 
contain  a  few  results  which  comport  with  the  evidence  preceding. 


RUBBER-CONTENT   BY   CHEMICAL   METHODS. 

The  analysis  of  the  guayule  plant  in  order  to  determine  its  rubber  and 
resin  content  presented  unexpected  difficulties,  but  the  results  attained, 
after  these  had  been  met,  are  undoubtedly  more  reliable  than  earlier 
analyses.  I  therefore  adopt  them  as  exposed  in  table  52  (Whittelsey, 
1909,  pp.  3,  5). 

TABLE  52. — Percentage  of  rubber  in  various  parts  of  guayule  shrub.    Field  plants. 


Parts. 

Rubber. 

Trunk  bark  
Root  bark             

per  ctnt. 
21-4 
IQ.S 

Branches  and  leaves  
Trunk  wood                

9-7 

o.o 

Root  wood  

2.0 

"The  percentage  of  pure  rubber  in  the  whole  trunk  is  9.9,  the  whole 
root  7.8,  the  branches  and  leaves  9.7,  and  in  the  whole  plant  9.5,  *  : 
based  on  perfectly  dry  material.  If  'mill  weight'  is  taken  as  a  basis,  the 
percentage  of  pure  rubber  in  the  whole  plant  is  7.8."  This  result  is  found 
to  correspond  very  closely  to  factory  experience  and  the  more  accurate 
published  results,  and  we  may  therefore  adopt  it  as  exact  enough  for  the 
present  purpose. 

The  only  figures  available  for  irrigated  plants  are  given  in  table  53 
on  the  following  page. 


186 


Guayule. 


TABLE  53 . — Analysis  of  irrigated  plant  two  years  old  from  transplanted  stocks,  Cedros. 
Collected  April  4,  1909.     Plant  weighing  4.5  pounds  fresh. 

{I)  The  original  stump  planted  March  1907,  divested  of  its  subsequent  growths. 
(II)  The  growth  of  1907  separated  into  wood  and  cortex:  the  wood  (Ila),  the 
cortex  (bark)  (116).  (Ill)  The  growths  of  1908  intact,  and  therefore  comprising 
both  wood  and  cortex.  (IV)  The  growth  of  1909,  consisting  of  short  new  twigs 
and  their  leaves,  developed  before  the  date  of  collection.  (V)  The  lateral  roots 
intact. 


Number. 

Rubber. 

Number. 

Rubber. 

I.. 

per  cent. 
3    55 

Ill 

per  cent. 
3   3° 

Ila  
116 

0.8o 
3    68 

IV  
V     .    ... 

0.67 
3   95 

The  method  by  which  the  above  data  were  obtained  was  worked  out 
by  my  former  colleague,  Dr.  Whittelsey.  The  method  was  controlled  by 
myself  microscopically,  and  the  material  was  found  after  treatment  to 
have  been  thoroughly,  though  not  quite  entirely,  extracted.  The  error 
from  this  source,  as  shown  by  this  control,  is,  however,  extremely  small, 
and  the  figures  may  be  accepted  as  practically  correct. 

For  the  purpose  of  appreciating  the  practical  significance  of  the  data, 
we  may  compare  the  percentage  of  rubber  in  the  new  growth  intact.  For 
field  plants  we  have  a  9.7  per  cent  rubber-content.  In  the  twigs  of  the 
irrigated  plants  studied  the  amount  is  3.3  per  cent,  namely,  a  little  over 
one-third  that  of  field  plants.  By  comparing  Ila  and  116,  we  note  that 
this  low  percentage  is  due,  as  shown  in  Chapter  V,  to  the  low  percentage 
of  rubber  in  the  wood  and  its  relatively  larger  volume  in  irrigated  plants. 
Moreover,  the  "branches  and  twigs"  of  Whittelsey 's  table  can  not  be 
directly  compared  with  those  of  III  in  my  own,  but  rather  with  II  and 
III  taken  together.  If  it  were  possible  to  compare  the  cortices  alone 
we  should  find,  in  all  probability,  a  percentage  of  about  4  per  cent  of  rub- 
ber for  irrigated  plants  against  15  to  20  per  cent  for  field  plants,  so  that 
for  the  new  growths  under  irrigation  from  the  transplanted  stocks  in 
question  the  amount  of  rubber  formed  by  cortical  tissues  is  about  one- 
fourth  to  one-fifth  of  that  formed  in  the  corresponding  tissues  in  the 
smaller  branches  and  twigs  of  field  plants.  But  the  rate  of  growth  under 
irrigation  is  such  as  to  result  in  the  production  of  a  volume  of  cortical 
tissues,  at  the  very  least  five  times  greater  for  the  same  length  of  time. 
This  factor  would  be  very  much  increased  if  field  and  irrigated  seedlings 
were  compared.  The  conclusion  would  therefore  appear  to  be  reached 
that  the  difficulty  attached  to  the  problem  of  cultivating  guayule  for  the 
rubber  is  not  that  of  obtaining  rubber,  but  of  properly  handling  the  raw 
material  so  as  to  extract  the  rubber  from  the  tissues. 

In  the  first  place,  we  have  repeatedly  noted  the  relatively  large  vol- 
ume of  the  wood  cylinder  in  irrigated  plants,  and  its  density.  We  have 
also  seen  that  the  branches  are  long  and  lithe.  If  this  material  is  handled 
in  its  entirety,  the  volume  of  barren  material  which  must  be  handled  by 
machinery  is  considerably  greater  than  in  the  case  of  field  plants.  The 
suggestion  (Whittelsey,  1909,  p.  6)  that  the  cost  of  manufacture  could 
be  reduced  by  the  use  of  decorticating  machinery,  as  is  done  in  the  case 
of  "grass  rubber"  (Funtumia  spp.)  in  Africa,  is  still  more  pertinent  for 


The  Origin  and  Occurrence  of  Rubber.  187 

irrigated  shrub,  and  the  character  of  the  growth  lends  itself  to  this.  This 
would  seem  to  be  necessary  in  the  event  that  the  relative  amount  of  rub- 
ber in  the  cortex  can  not  be  raised  above  3.5  to  4  per  cent,  not  only  because 
of  this  probable  difficulty  of  agglomerating  the  more  finely  divided  rubber, 
but  because  of  the  interference  with  this  of  the  fragments  of  splintery 
wood,  which  will  tend  materially  to  obstruct  agglomeration  in  any  event. 

In  the  second  place,  the  individual  masses  of  rubber  in  the  irrigated 
plant  are  smaller  and  further  away  from  each  other  than  in  field  plants. 
Hence,  as  above  said,  it  is  more  difficult  to  agglomerate  the  rubber.  This 
is  noted  in  trying  to  isolate  the  rubber  from  irrigated  tissues  by  mastica- 
tion, a  process  more  difficult  than  for  field  plants.  It  may  be  found  neces- 
sary to  introduce  a  machine  especially  adapted  to  mastication  of  the 
material  after  passing  through  the  pebble-mill,  in  which  rollers  with  differ- 
ential speeds  will  cause  the  massing  of  the  minute  particles  of  rubber.  But 
the  practical  solution  of  such  problems  is  not  to  be  obtained  merely  by 
reasoning  about  them.  The  laboratory  and  factory  are  mutually  of  value, 
but  the  one  does  not  always  solve  the  difficulties  of  the  other. 

VARIATION  IN  RELATIVE  AMOUNT  OF  RUBBER  IN  FIELD 
PLANTS. 

I  have  already  pointed  out  that  rubber  does  not  appear  in  newly 
formed  tissues  for  some  time  after  the  cessation  of  growth ;  it  may  be  for  a 
period  of  some  months.  It  therefore  appears  that  the  new  growth  of  field 
plants  taken  at  some  periods  of  the  year  has  a  content  and  distribution 
of  rubber  similar  to  that  in  irrigated  plants,  aside  from  the  relative  bulk 
of  the  tissues  themselves.  To  illustrate,  I  take  the  following  analysis  of 
seedlings,  from  Station  2,  Quadrat  4  (plate  17,  fig.  A), collected  April  1909, 
germinated  in  1908  (table  54).  The  leaves  and  stems  with  tap-roots  were 
analyzed  separately. 

TABLE  54. 


Rubber. 

The  leaves  
The  stem  and  tap-roots.  .  .  . 

per  cent. 

I  .21 

2.40 

Of  interest  in  this  table  are  the  rubber-content  of  the  leaves  taken 
separately  and  the  low  content  of  the  stems  and  tap-roots.  The  leaves 
probably  represent  the  usual  condition,  as  they  were  old,  fully  matured 
leaves  which  had  remained  attached  to  the  plants  throughout  a  long 
drought  period.  The  plants,  however,  were  of  rapid  growth,  indeed 
remarkably  rapid  for  field  plants,  and  the  low  rubber-content  stands  in 
relation  to  this.  There  is  no  doubt  that  this  rubber-content  is  much  lower 
than  for  seedlings  of  the  same  size  of  slow  growth. 

In  this  respect,  therefore,  there  is  no  hard  and  fast  difference  as 
between  field  and  irrigated  plants,  nor  indeed  is  this  the  case  for  the  relative 
volumes  of  the  tissues  themselves,  as  I  have  previously  shown  (p.  117). 
The  response  of  the  guayule  under  irrigation,  therefore,  is  but  an  extreme 
expression  of  what  occurs  in  nature,  correlated  with  the  climatic  differ- 
ences which  obtain  from  year  to  year,  and  in  different  localities. 


188  Guayule. 

RELATION  OF  RUBBER  AND  RESIN. 

A  notion  has  been  widely  entertained  that  the  amount  of  rubber  in 
the  guayule  plant  is  in  some  way  related  to  the  amount  of  resin.  This 
naturally  grew  out  of  the  fact  that  commercial  rubbers  always  contain 
more  or  less  resin,  and  that  resin  is  abundant  in  the  guayule.  In  the 
preparation  of  the  commercial  article  from  the  guayule  the  resin  becomes 
intermingled  with  the  rubber  to  the  amount  of  20  per  cent  (Whittelsey, 
1909).  There  appears,  however,  to  be  no  adequate  evidence  in  support 
of  this  notion,  while  on  the  other  hand  there  is  strong  evidence  to  show  that 
the  physiological  processes  involved  in  the  secretion  of  these  two  materials 
are  quite  distinct: 

1 .  The  canals  which  are  laid  down  in  the  protogenic  tissues  become 
actively  secreting  as  regards  resin  long  before  rubber  appears  at  all.    This 
is  strikingly  evident  in  irrigated  plants,  in  which  the  amount  of  growth 
is  very  much  in  excess  of  that  in  field  plants. 

2.  Resin  is  secreted  in  other  Compositae  in  which  rubber  does  not 
occur.     In  the  closely  related  mariola   (Parthenium  incanum)  resin  is 
abundant,  while  rubber  is  very  meager  in  amount;  and  this  is  true  of 
many  others. 

3 .  In  irrigated  plants  the  amount  of  resin  is  correlated  with  the  ana- 
tomical conditions  within  the  organism,  while  the  secretion  proper  appears 
to  be  neither  retarded  nor  advanced  by  the  presence  of  water.    Water,  on 
the  other  hand,  affects  markedly,  though  probably  indirectly,  the  rate  of 
rubber  secretion,  which  lags  behind  growth.     But  the  lagging  behind  of 
rubber  secretion  is  not  in  inverse  relation  to  any  possible  increase  which 
may  be  shown  to  occur  in  the  secretion  of  resin. 

4.  The  distribution  of  starch  appears  to  be  connected  with  the  secre- 
tion of  resin,  as  in  other  well-known  instances  (e.g. ,  Pinus] .    The  secretion 
of  resin  appears,  as  above  pointed  out,  to  be  extra-protoplasmic,  and  in 
harmony  with  the  view  expressed  by  Tschirch,  already  alluded  to. 

5.  Rubber,  however,  appears  in  the  tissues  independently  of  the  dis- 
tribution of  starch  referred  to  in  (4)  above.    However,  the  starch  found 
in  the  young  tissues  near  the  growing  apex  may  serve  as  a  source  of  ma- 
terial for  the  elaboration  of  rubber. 

6.  The  appearance  of  rubber  in  the  canal-cells  might  be  cited-to  sup- 
port the  view  under  discussion,  but  for  the  fact  that  the  rubber  is  merely 
accumulated  in  these  cells  and  that  this  occurs  later  than  the  secretion 
of  resin.    Further,  rubber  occurs  in  other  tissues,  e.g.,  parenchyma  rays, 
far  removed  from  resin  secretion.    Resin  in  the  canal-cells  has  not  been 
demonstrated,  but  in  the  meatus  only. 

THE  SIGNIFICANCE  OF  RUBBER. 

The  inevitable  question  as  to  the  use  of  rubber  to  such  a  plant  as  the 
guayule,  subject  as  it  is  to  the  severe  conditions  of  the  desert,  has  been 
raised  and  must  be  met  in  some  wise.  I  have  already  briefly  discussed 
the  matter  (Lloyd,  1909)  with  but  meager  satisfaction,  as  will  appear  to 
those  inclined  to  find  a  use  for  everything  in  animate  nature.  I  can  only 
repeat  here  what  I  have  already  said. 


The  Origin  and  Occurrence  of  Rubber.  189 

The  most  obvious  suggestion  relates  to  the  conservation  of  water, 
and  it  seems  quite  possible  that  the  rubber  may  act  as  a  sort  of  blanket, 
reducing  to  some  extent  the  passage  of  water  to  the  outer  zones  of  tissue 
and  consequently  to  the  outside  of  the  plant,  and  as  a  storage  material. 
The  slower  deposition  of  rubber  in  irrigated  plants  and  its  behavior  in 
Castilloa  elastica  under  similar  circumstances  lend  a  modicum  of  support 
to  this  view.  Rubber,  as  is  well  known,  will  take  up  and  retain  a  certain 
amount  of  water  with  considerable  tenacity.  One  would  be  encouraged 
to  hold  this  view  if  rapidly  grown  field  seedlings  with  much  less  than  the 
normal  amount  of  rubber  had  not  been  known  to  pass  successfully  through 
a  long  period  of  drought,  indeed  much  longer  than  usual.  Further,  mari- 
ola  appears  to  be  as  well  equipped  for  resisting  drought  as  guayule,  but 
contains  a  very  small  amount  of  rubber.  The  obvious  objection  that  the 
mariola  has  some  other  means  to  the  end  would  in  this  case,  I  believe,  be 
difficult  to  demonstrate,  and  as  difficult  to  refute.  We  are  here  in  the 
field  of  teleological  speculation. 

Spence  (1908),  studying  latex,  found  that  this  contains  oxidases 
capable  of  acting  upon  caoutchouc,  and  argued  that  this  material  may 
therefore  serve  as  a  reserve  food  material.1  Similar  enzymes  probably 
occur  in  the  guayule,  but  it  is  safe  to  remark  that  in  this  plant,  once  the 
rubber  is  laid  down,  it  is  there  to  stay,  as  shown  by  its  abjection  in  com- 
pany with  the  bark-tissues.  Even  in  the  cells  adjacent  to  the  active  cam- 
bium, or  other  physiologically  active  tissues,  the  amount  is  never  reduced, 
while,  if  of  use  as  a  source  of  energy  to  the  growing  twigs,  we  should  find 
some  evidence,  analogous  to  that  seen  in  the  starch-content  of  growing 
twigs,  that  there  is  translocation.  But  such  evidence  is  quite  lacking. 
Whatever  may  prove  to  be  true  of  latex  plants,  therefore,  there  does  not 
appear  to  be  the  slightest  evidence  that  rubber  is  in  any  sense  a  food 
material  in  the  guayule. 

This  view  has  recently  (1909)  been  again  brought  into  question  by 
Spence: 

The  fact  that  the  caoutchouc,  or  rubber,  does  not  occur  in  any  definite  latex 
system  in  the  guayule,  but  in  the  parenchymatic  cells  of  the  medullary  rays  and 
cortex,  and  further,  that  the  amount  of  rubber  from  the  dried  plant  varies  con- 
siderably from  one  period  of  the  year  to  the  other  *  *  *,  seems  at  once  to 
suggest  to  my  mind  that  the  rubber  must  have  an  important  function  in  meta- 
bolic processes.  That  the  rubber  is  cast  off  partially  and  in  a  modified  form  in  the 
bark,  as  Professor  Lloyd  has  pointed  out,  does  not  in  any  way  weaken  the  evidence 
of  my  theory,  and  from  experiments  which  I  have  recently  made  I  have  found 
that  young  Ficus  elastica  trees,  grown  in  an  atmosphere  and  soil  free  from  carbon 
dioxide,  gradually  drew  upon  their  milk,  which  became  nothing  more  than  water 
after  a  few  weeks'  time.2  In  any  case  *  *  *  the  guayule  plant  shows  very 
clearly  that  we  can  hardly  retain  the  theory  that  the  latex  merely  affords  protec- 
tion to  the  plant  against  internal  injury  and  moisture  in  time  of  drought;  in 
guayule  there  is  no  secretion  on  injuring  the  plant,  and  no  reserve  water-supply, 
though  the  rubber  is  there  all  the  time.  *  *  *  " 

1  See  also  Cook,  1903. 

2  There  has  been  a  long  controversy  on  the  function  of  latex,  for  an  account 
of  which  see  Tschirch,  1906. 

3  The  quotation  was  printed  in  the  past  tense  and  third  person.     ] 
made  it  into  the  first  person,  present.    The  italics  are  Spence's.    (Lloyd,  1909.   Dis- 
cussion, p.  141). 


190  Guayule. 

Dr.  Spence  adds  that  sugars  are  to  be  found  in  quantities  in  certain 
barks,  and  that  the  physiological  importance  of  these  can  not  be  doubted. 

The  answer  would  seem  to  be  that  whatever  occurs  in  Ficus  elastica 
can  only  be  of  suggestive  value  with  regard  to  the  guayule.  And  the 
behavior  of  sugar  described  means  that  the  unused  residue  of  the  sugar 
has  been  cut  out  by  periderm,  just  as  the  unused  portion  of  any  other  sub- 
stance may  be.  But  this  can  not  mean  that  everything  which  appears  in 
the  bark  must  have  been  of  use  to  the  plant.  The  statement  made  by  me 
that  the  amount  of  rubber  varies  from  time  to  time  in  the  year  does  not 
mean  that  the  absolute  amount  in  a  particular  individual  is  now  reduced 
and  now  increased.  It  means  that  the  amount  of  rubber  relative  to  the 
weight  of  the  plant  is  greater  at  one  time  than  another,  and  I  myself  have 
shown  this  to  be  the  case.  The  gradual  accumulation  in  the  tissues,  unac- 
companied by  any  reduction,  of  rubber  which  might  serve  a  storage  func- 
tion, this  accumulation  following  growth,  seems  to  completely  contradict 
the  view  that  rubber  is  a  reserve  food.  We  may  very  well  say  that  during 
growth  energy  is  diverted  from  the  secretion,  or,  as  I  should  prefer  to  say, 
excretion,  of  rubber,  and  this  would  accord  with  the  fact  that  the  more 
energy  is  expended  in  growth  the  slower  the  secretion  takes  place. 

In  the  statement  to  which  Dr.  Spence  refers,  when  I  speak  of  rubber 
being  cast  off  in  a  modified  form  I  do  not  mean  to  say  that  this  modifi- 
cation is  chemical,  or  that  it  takes  place  before  the  rubber  is  cast  off, 
but  by  virtue  of  (presumably)  oxidizing  processes  which  take  place  in  the 
cork-cells,  which  are  now  dead.  This  change,  it  seems  to  me,  can  have,  in 
the  light  of  the  evidence,  no  significance  to  the  plant.  It  remains,  how- 
ever, to  show  experimentally  that  my  view  is  correct,  but  it  can  scarcely 
be  denied  that  the  evidence  against  it  is  tenuous. 

SUMMARY. 

The  studies  presented  in  this  chapter  may  be  summarized  as  follows : 

1 .  In  the  root,  rubber  is  first  secreted  in  the  primary  canal-cells  (plate 
41,  fig.  6),  the  activity  spreading  from  this  region  as  a  center,  but  more 
rapidly  along  the  radius.    At  about  the  same  time,  or,  judging  from  the 
size  of  the  granules  seen,  somewhat  later,  it  appears  in  the  innermost  cells 
of  the  parenchyma  rays.     Rubber  appears  in  the  root  earlier  than  in  the 
stem  in  the  same  plant. 

2.  Accumulation  usually  takes  place  in  the  oldest  cells  first;  that  is, 
those  in  the  outer  zones.    Thus,  in  the  root  the  primary  cortex  contains, 
before  the  maximum  content  for  all  the  cells  has  been  attained,  more 
rubber  than  the  cells  of  the  secondary  cortex;  and  the  outer  cells  of  the 
latter  contain  more  than  the  inner.     Accumulation  (in  irrigated  plants 
at  least)  is  more  rapid  in  the  parenchyma-ray  cells  than  either  in  the  pith 
or  the  cortex. 

In  the  primary  cortex  of  the  stem  rubber  may  never  appear,  as,  e.g., 
in  irrigated  plants  in  which  growth  and,  hence,  secondary  changes  are  so 
rapid  that  the  primary  cortex  does  not  have  time  enough  for  secretion. 

3.  With  one  exception,  namely,  in. the  hypocotyl,  the  accumulation 
of  rubber  in  the  stem  takes  place  earlier  in  the  pith  than  in  the  paren- 
chyma rays  or  cortex,  and  earlier  in  the  rays  than  in  the  cortex. 


The  Origin  and  Occurrence  of  Rubber.  191 

At  the  apex  of  the  stem  of  field  plants  more  rubber  is  found  in  the 
pith  than  in  the  cortex  after  prolonged  drought. 

In  the  hypocotyl  (upper  zones)  accumulation  of  rubber  takes  place 
more  rapidly,  if  not  earlier,  in  the  inner  parts  of  the  parenchyma  rays. 
This  appears  to  be  due  to  a  more  primitive  physiological  condition  of  the 
pith  of  the  hypocotyl. 

4.  With  questionable  exceptions,  the  accumulation  of  rubber  is 
earlier  in  the  "  secreting-cells  "  of  the  resin-canals  than  in  the  surrounding 
tissues.    The  exceptions  noted  were  (a)  in  the  apex  of  a  very  slowly  grown 
field  seedling,  in  the  resin-canals  of  which  no  rubber  was  noted,  and  (6) 
in  the  new  twigs,  near  the  apex  of  field  plants.    Rubber  may  be  noted, 
however,  in  the  canal-cells,  as  in  a  very  rapidly  grown  irrigated  seedling, 
though  it  occurs  nowhere  else. 

5.  The  amount  of  rubber  in  the  cells  of  small  seedlings l  in  the  field  is 
relatively  as  great,  or  very  nearly  so,  as  in  mature  plants,  except  in  those 
seedlings   (table  54)  which  have  grown  rapidly  in  the  field,  and  which 
have  not  had  sufficient  time  for  the  accumulation  of  the  full  complement 
of  rubber. 

6.  Rubber  occurs  unchanged  in  the  portions  of  the  secondary  cortex 
which  have  been  more  recently  cut  out  by  inner  periderm.    In  the  cells 
arising  directly  from  the  outer  or  inner  periderm  rubber  does  not  occur. 
In  the  bark  proper  the  rubber-bearing  tissues  alternate  with  nearly  bar- 
ren suber.    Volume  for  volume,  therefore,  the  bark  contains  less  rubber 
than  the  contingent  living  cortex  which  still  remains  unmodified. 

7.  Rubber  occurs  in  the  pseudotylose  tissue  of  the  resin-canals  in 
quantities  comparable  to  the  amount  found  in  adjacent  cells. 

8.  The  accumulation  of  rubber  in  the  new  tissues  of  secondarily 
thickened  roots  and  stems  is  analogous  to  that  in  those  still  in  the  primary 
condition.    It  is  for  some  time  absent  from  the  newer  parts  of  the  paren- 
chyma rays,  and  secretion  occurs  first  in  the  innermost  and  outermost 
cells  simultaneously.    The  march  of  the  secretion  of  rubber  is,  therefore, 
from  the  base  toward  the  tip  of  new  shoots  and  from  the  pith  and  cortex 
toward  the  cambium  in  older  stems. 

9.  In  field  plants,  that  is,  in  those  subjected  to  the  usual  desert  con- 
ditions of  their  habitat,  the  accumulation  of  rubber  is  more  rapid  than  in 
irrigated  plants.    The  maximum  quantity  is  certainly  not  reached  in  four 
months  (June  to  September,  incl.,  1908)  after  growth  commences,  and  it 
is  highly  probable  that  six  or  more  months  must  elapse. 

In  a  given  cell,  the  amount  of  rubber  in  a  field  plant  will  generally  be 
greater  at  the  end  of  one  year  than  in  a  corresponding  cell  in  the  irrigated 
plant  in  two  years.  Also,  cells  containing  a  given  quantity  of  rubber  will 
be  found  nearer  the  apex  of  the  stem  of  field  plants  than  of  irrigated  plants. 
It  is  probable,  again,  that  the  total  amount  of  rubber  that  a  cell  in  a  field 
plant  is  capable  of  secreting  is  greater  than  in  an  irrigated  plant,  though 
this  is  not  certain. 

1  Chemical  analyses  of  entire  small  seedlings  are  misleading,  because  of  (a) 
the  larger  relative  bulk  of  the  leaves,  and  (6)  the  greater  relative  volume  of  tissues 
partially  filled  with  rubber,  as  in  the  case  of  seedlings  taken  after  a  period  of  growth, 
but  before  the  maximum  rubber-content  has  been  reached. 


192  Guayule. 

The  determination  of  the  time  at  which  the  maximum  rubber  con- 
tent is  reached  is  of  economic  importance,  as  the  earlier  gathering  of  shrub 
involves  a  considerable  economic  loss,  amounting  approximately  to  the 
quantity  of  rubber  secreted  in  one  year  in  the  new  parts.  If  consistent, 
therefore,  with  other  considerations,  the  gathering  of  shrub  should  not 
occur  during,  or  for  some  time  after  the  close  of,  the  growing  season.  It 
will  be  understood  that  by  new  parts  is  meant  the  new  tissues  within  the 
already  secondarily  thickened  roots  and  stems,  as  well  as  new  accretions 
in  length.  The  time  at  which  the  maximum  amount  of  rubber  may  be 
expected  differs  with  the  length  of  the  growing-season,  which  depends 
upon  the  rainfall  and  the  intensity  of  the  drought  following.  It  thus 
happens  that  field  conditions  are  sometimes  such  as  to  produce  results  in 
field  plants  (seedlings,  table  54)  similar  to  those  in  irrigated  plants. 

10.  The  rate  at  which  rubber  is  secreted  by  irrigated  plants,  under 
the  conditions  described  for  the  Cedros  experimental  plants,  is  such  that 
at  the  close  of  the  second  season's  growth  (Sept.  1908),  the  amount  in 
the  cells  is  sufficient  to  agglomerate  into  the  large  masses  characteristic  of 
field  plants.    This  condition  was,  however,  approached  after  a  succeeding 
drought-period  lasting  till  April  1909  (plate  43,  figs,  i  and  2;  table  53). 
In  plants  grown  at  Caopas  under  irrigation,  during  the  first  season's 
growth  (1908)  and  with  a  restricted  amount  of  water  during  the  second 
season  (1909),  the  amount  of  rubber  was  evidently  greater  than  in  the 
Cedros  material  *  and  was  great  enough  by  October  to  agglomerate  (plate 
43,  figs.  3,  4),  forming  dense  masses,  but  not  as  large  as  in  field  plants. 
There  is,  however,  a  large  enough  rubber-content  in  such  plants  for 
mechanical  extraction,  though  it  is  probable  that  some  adaptation  of  the 
process  would  be  necessary.    Although  the  amount  of  rubber  may  be  as 
low  as  3  per  cent,  it  must  not  be  forgotten  that  the  rate  of  growth  under 
irrigation  is  enormously  in  excess  of  that  under  field  conditions. 

1 1 .  There  appears  to  be  no  direct  physiological  relation  between  the 
secretion  of  rubber  and  of  resin. 

12.  Rubber  appears  to  have  no  physiological  function  in  the  guayule 
plant. 

1  The  slowness  of  secretion  in  well-watered  plants  offers  an  interesting  analogy 
to  the  behavior  of  the  rubber-bearing  latex  plant  Castilloa  elastica.  (Collins,  and 
Pittier ;  see  Cook,  1 903) .  Olsson-Seffer  has  also  pointed  out  that  the  secretion  of  rub- 
ber in  this  plant  is  retarded  by  irrigation,  and  in  consequence  it  must  be  deprived 
of  water  for  some  time  before  it  can  be  tapped  to  advantage. 


LLOYD 


PLATE  43 


1.  Base  of  1908  growth,  August.     Cedros,  irrigated. 

2.  Growth  of  1908  in  April  1909.     Cedros,  irrigated. 

3.  Cortex,  2-year  old  stem.     October  1909.     Caopas,  irrigated. 

4.  Pith  of  same  plant. 

5.  Epidermis  of  an  old  leaf,  field  plant,  April  1909. 


CHAPTER  VIII. 
VEGETATIVE  REPRODUCTION. 

In  attempting  to  solve  the  problem  of  the  cultivation  of  a  hitherto 
totally  feral  desert  plant,  it  became  necessary  to  determine  quantitatively 
the  possibilities  of  the  plant  for  reproduction  vegetatively  as  well  as  by 
seed.  As  has  been  mentioned,  the  percentage  of  germination  is  small, 
even  under  the  best  cultural  conditions,  so  that  any  haphazard  field 
method  of  sowing  seed,  in  the  hope  that  nature  will  do  the  rest,  is  prac- 
tically out  of  the  question.  In  the  hope  that  cuttings  could  be  made  to 
grow  readily  and  in  sufficiently  large  quantities  for  cultural  purposes,  this 
was  gone  into  thoroughly.  The  net  result  of  all  the  experiments  is  to 
show  that  only  a  short  zone  of  the  stem  is  capable  of  root-regeneration, 
namely ,  that  immediately  above  the  tap-root,  but  including  some  portion, 
difficult  to  delimit,  of  the  epicotyledonary  region  in  seedlings  and  an  anal- 
ogous portion  of  the  stem  in  retonos  (fig.  n).  The  ability  to  produce 
roots  in  plants  from  seed  is,  however,  not  restricted  to  the  main  stem,  but, 
as  will  be  shown,  resides  also  in  branches  springing  from  the  root-producing 
zone.  This  fact  is  of  rather  special  biological  as  well  as  economic  interest, 
and  as  it  throws  light  on  the  failure  of  attempts  to  grow  cuttings  I  shall 
first  present  my  observations  leading  to  the  conclusion  stated. 

INDUCED  ROOT-REGENERATION.1 

Both  the  Mexican  guayule  (Parthenium  argentatum  A.  Gray)  and  its 
congener,  the  mariola  (P.  incanum  H.  B.  K.),  exhibit  methods  of  vegeta- 
tive reproduction  which,  while  shared  by  other  plants,  are  not  common  to 
these  under  the  normal  conditions  of  growth.  A  somewhat  detailed  ac- 
count of  the  matter  has  already  been  published,2  but  a  brief  restatement 
will  be  necessary  to  make  clear  the  point  of  the  present  discussion. 

The  mariola  is  a  low  shrub  with  rather  numerous  branches  rising 
immediately  from  the  base  of  the  chief  stem.  These  branches  arise  sub- 
sequently to  the  development  of  the  chief  shoot,  and  not  unusually,  during 
the  first  season  of  growth,  from  the  seedling.  Each  following  period  of 
development  sees  new  lateral  shoots  of  this  kind  arise  again  from  the  base, 
either  of  the  main  shoot  or,  secondarily,  from  an  already  well-developed 
basal-lateral  shoot.  Long  continuation  of  this  process  results  in  the  dense 
group  of  stems  arising  near  the  surface  of  the  ground  which  characterizes 
the  mature  plant  of  the  mariola. 

It  is  to  be  further  noted  that  nearly  all  of  these  basal-lateral  shoots 
are  provided  with  their  own  root-systems  (plate  44,  fig.  B).  From  the 
base  of  each  new  shoot,  soon  after  it  has  accomplished  a  fair  amount  of 
development,  there  spring  adventitious  roots,  one  of  which,  by  the  direc- 

1  Presented  before  Section  G  of  the  American  Association  for  the  Advance- 
ment of  Science  at  the  Baltimore  meeting,  1908. 

2  Lloyd,  19086. 

13  193 


194  Guayule. 

tion  and  amount  of  growth,  becomes,  in  effect,  a  tap-root  of  the  branch 
from  which  it  springs.  Subsequent  development  of  roots  of  the  second 
and  higher  orders  results  in  the  ultimate  elaboration  of  a  complete  root- 
system. 

We  find  furthermore  that,  while  the  caliber  of  the  basal-lateral  stem 
increases  with  age,  the  isthmus  of  tissue  between  this  and  the  chief  stem 
increases  only  slowly,  so  that  there  is  never  more  than  a  weak  connection 
established,  and  this  ultimately  becomes  disintegrated.  In  this  manner 
the  basal  branches  in  question  are  set  free  from  the  parent  stock.  There 
results,  therefore,  from  a  single  original  stock,  a  group  of  independent 
individuals  closely  crowded  together. 

A  departure  from  this  behavior  is  sometimes  to  be  found.  A  glance 
at  the  root-system  of  a  single  stock  will  show  that  the  lateral  roots  run  ob- 
liquely into  the  soil,  so  that  they  soon  attain  a  considerable  depth.  From 
the  upper  portion  of  these  lateral  roots  retonos  occasionally  arise  which 
behave  much  as  do  the  basal-lateral  branches  above  described,  and  the 
net  result  is  the  same,  namely,  to  produce  a  crowded  group  of  individual 
plants. 

The  root-system  of  the  guayule,  on  the  other  hand,  consists  of  a 
strong  tap-root  and  several  strong  laterals,  which  arise  at  a  short  distance 
below  the  surface  of  the  soil  (plate  9,  fig.  A).  These  follow  a  horizontal 
path  for  a  distance,  it  may  be,  of  2  meters  or  more  from  the  plant,  and  con- 
stitute a  water-collecting  system  by  which  the  plant  derives  water  from 
rain-water  which  does  not  penetrate  deeply — a  feature  shared  by  many 
desert  plants  (Cannon,  1911).  These  shallow  roots  frequently  produce 
root-shoots  (retonos)  at  various  distances  from  the  parent  stock.  I  have 
found  them  at  a  meter  distant,  and  it  is  likely  that  they  may  arise  still 
farther  away,  though  I  believe  less  often  than  at  shorter  distances. 

It  may  be  presumed  that  shoots,  arising,  as  they  not  infrequently  do, 
from  the  basal  portion  of  the  main  axis,  may  occasionally  strike  root  as  in 
the  mariola.  Many  thousands  of  plants,  however,  have  been  examined, 
and  only  one  or  two  cases  have  been  found  which  may  be  permitted  this 
interpretation.  We  may  therefore  regard  the  method  described  as  the 
only  normal  method  of  vegetative  reproduction  under  natural  conditions, 
though  it  has  been  observed  to  occur  in  the  field  (Station  5)  in  two  cases 
in  which  the  aerial  portion  of  the  plant  had  been  removed. 

On  observing  for  the  first  time  the  conditions  above  described  in  the 
mariola,  it  occurred  to  me  that  it  ought  to  be  possible  to  induce  the  guayule 
to  behave  similarly.  The  fact  that  a  guayule  retono  strikes  new  adven- 
titious roots  from  its  basal  zone  (fig.  i'i),  and  that  this,  in  common  with 
that  part  of  the  chief  axis  above  the  cotyledons,  has  a  different  ana- 
tomical structure  from  other  stems,  gave  color  to  the  notion  that  there 
are  physiological  grounds  for  entertaining  the  belief  that,  with  proper 
treatment,  the  possibility  might  be  realized. 

As  experiments  to  this  end  would  have  necessarily  involved  a  long 
period  of  time,  it  was  fortunate  that  I  had  under  observation  at  Cedros 
plants  which  had  been  growing  for  the  major  part  of  two  seasons  under 
irrigation.  This  was  in  September  1908.  These  plants  had  grown  from 
stumps  which  were  planted  in  March  1907,  by  Mr.  C.  T.  Andrews.  The 


Vegetative  Reproduction.  195 

parent  plants  had  been  taken  from  an  old  stack-ground  in  Saltillo,  at  the 
guayule  factory  of  Martin  Brothers,  and  had  started  there  from  seed 
which  had  fallen  from  the  stacked  guayule.  Before  being  transplanted, 
they  were  variously  trimmed  back,  leaving  only  the  lower  portions  of  the 
main  stem  and,  in  some  instances,  of  the  lowermost  branches.  During 
1907  the  new  growths  attained  a  length  of  about  25  cm.,  making  rounded 
bushes  about  15  cm.  in  diameter.  By  September  1908  another  25  cm. 
of  growth  brought  them  to  a  spread  of  a  meter  for  the  largest  plants. 

It  was  then  discovered  (on  the  1 9th  of  September)  that  the  lowermost 
new  shoots  in  certain  of  these  plants  had  struck  root,  quite  after  the 
manner  described  for  the  mariola,  and  it  was  further  observed  that  this 
had  not  occurred  in  all  of  the  plants,  but  either  in  those  plants  which  had 
been  trimmed  back  so  as  to  leave  only  a  very  short  basal  portion,  or  in 
those  new  shoots  which  had  arisen  close  to  the  tap-root  (plate  44,  fig.  A). 
In  several  instances  the  whole  of  the  lowermost  branch  was  buried  by 
chance  in  the  soil,  and  in  others  a  part,  but  neither  in  these  nor  in  some 
layering  experiments  by  Dr.  Kirkwood J  was  any  response  observed.  The 
behavior  of  guayule  in  this  respect  is  similar  to  that  of  certain  plants 
which  are  subjected  to  mound-layering.  Whether  it  is  possible  to  compel 
every  plant  properly  treated  to  behave  in  the  manner  described  can  not 
be  said,  as  circumstances  prevented  a  more  careful  study  of  the  matter.2 
If  this  should  prove  the  case,  it  is  evident  that  the  branches  which  are  pro- 
vided with  their  own  root-systems  could  be  removed  and  transplanted 
with  ease. 

PROPAGATION    BY  CUTTINGS. 

The  general  conclusion  suggested  by  the  above  experience  was  that 
only  cuttings  taken  from  the  root,  or  from  a  portion  of  the  stem  near  the 
top  of  the  tap-root,  would  succeed,  but  as  the  time  of  my  stay  at  Cedros 
had  drawn  to  a  close  it  was  not  possible  to  direct  experiments  to  test  the 
latter  of  the  alternatives.  Table  55,  summarizing  the  results  of  my  study 
of  cuttings,  did  not  include  this  particular  condition,  which  could  hardly 
have  been  anticipated.  I  early  found,  however,  that  the  stem-cuttings 
made  did  not  respond,  and  that  recourse  must  be  had  to  cuttings  in  which 
a  portion  of  root-tissue  was  involved.  The  scheme  of  splitting  the  butt  of 
the  plant  so  as  to  get  two  to  four  pieces  was  seized  upon,  the  only  method 
of  those  used  which  secured  positive  results  aside  from  pure  root-cuttings. 

The  following  conclusions  may  be  drawn  from  the  data  in  table  5  5 : 

1.  Cuttings  involving  stem-tissues  alone,  with  a  possible  exception  of 
stem-tissue  close  to  the  root  in  seedling  or  rotono,  do  not  regenerate  roots 
under  the  treatment  given.    It  remains  theoretically  possible,  by  special 
and  more  refined  methods,  to  induce  root-regeneration,  but  for  the  pur- 
poses toward  which  the  experiments  were  chiefly  directed,  this  is  not 
practicable. 

2.  Stem-cuttings  may  be  kept  alive,  after  being  planted,  for  a  con- 
siderable period,  particularly  during  the  cooler  season,  by  using  careful 

1  Exp.  181,  182,  in  which  either  branches  or  whole  plants  were  layered. 

2  I  have  noted  the  same  behavior  in  guayule  from  Texas  planted  by  me  at  the 
Desert  Botanical  Laboratory  and  at  Auburn,  Alabama. 


196 


Giiayule. 


TABLE  55. — Experiments  in  propagation  by  cuttings. 


Exp. 
No. 

Date. 

Parts  used. 

No. 
of  cut- 
tings. 

Conditions. 

Results. 

1-6 

1907 
Aug.     2 

Stem  cuttings  of  vari- 

6s 

Set  vertically  or  hori- 

Negative. 

ous  ages. 

zontally   in    trays, 

watered. 

7 

Aug.    2 

Root  cuttings  2  to  5 

25 

Laid  horizontally  

Negative. 

cm.  long. 

8-1  s 

Aug.    3 

Various       parts      of 

116 

Garden  soil,  sand  and     Negative. 

stem. 

soil,  manured  soil. 

16—17 

Aug.     3 

Root  cuttings  from  ra- 

20 

Laid  horizontally  Negative. 

83 

Oct.   25 

pidly  grown  plants. 
Lateral  roots  of  field 

2 

Laid  horizontally,    Dec.  3,  new  shoots  i  to 

plants. 

lightly  covered  '       10  mm.  long,  on  one. 

with    soil    (garden  j       Dec.    24,    shoots    on 

soil).                                  both.    No  new  roots. 

1908 

Died  later. 

1300 

Jan.    24 

Field    plants.      1907 
growth  pinched  off, 
leaves  trimmed. 

100 

Planted  in  limestone     Apr.  6,  12  still  alive,  but 
soil  in  i-inch  paper        all    died    later.      No 
tubes    in    tray.         roots  formed. 

Transplanted  Apr. 

6,    into     prepared 

bed     of    limestone 

soil,    watered    and  , 

shaded. 

1306 

Jan.    24 

Ditto,  roots.  ... 

40 

Ditto  

Mar.  3,  2  buds  on  one 

cutting.      Apr.    6,    9 

living.  2  more  started 

after     transplanting. 

Aug.    28,   3    growing 
well. 

I30C 

Jan.    24 

[Ditto,    1907    growth 
cut  off  and  leaves 

100 

Ditto  

Apr.  6,   13  alive.     All 
died  later,   no   roots 

I3od 

Jan.    24 

trimmed  off. 
i.Ditto,  twigs  2  to  3 

40 

having  been  formed. 
Ditto  Apr.  6,  23  alive,  but  all 

years  old. 

died  later.    No  roots. 

130* 

Jan.    24 

Ditto,  1907    growth 
pinched     off      and 

20 

Ditto 

Apr.  6,  all  dead.     No 
roots. 

leaves  on  left. 

nof 

1 

Jan.    24 

Ditto,    cut    off    ob- 

IOO 

Ditto  Aor.   6.    21   alive.     All 

liquely  thro  ugh 

died  later.    No  roots. 

growth     of     1906. 

Leaves   trimmed 

131 

Jan.    27 

away. 
[Field  plants,  growth 
of  1907  broken  off; 
leaves  trimmed 

40 

Moist  sand  

Jan.  si,  all  dying,  rot- 
ting off. 

away. 

144 

Feb.     9 

Twigs   2   to   3    years 
old,  field  plants. 

IO 

Water  

Apr.  25,  all  dead. 

i48a 

Feb.  24 

"Root-shoot"     cut- 
tings;    lower  part 
of  stem  and  upper 

31 

Prepared    bed    lime- 
stone soil. 

Apr.  25,  growing  vigor- 
ously ii,  starting  8, 
well  started  but  wilt- 

part   of    tap-root, 

ing,  saved  by  heaping 

split   into    2    to    4 

soil    about   them    2. 

pieces. 

Started  well  but  died 

later  i  ;  failed  to  start 

9.    May  2,  one  more 
started  below  surface 

of  soil;  May  19,  an- 

other,    which     later 

died.    Aug.  28,  25  to 

32  cm.  growth  in  the 

I48& 

Feb.   24 

Roots 

Ditto 

above  living  cuttings. 
Both  started,  one  dying 

after  making  3   cm. 

growth. 

153 

Apr.     4 

Twigs  3  to  5  years  old. 
Leaves    trimmed 

3S 

Ditto,  with  shade  of 
cloth. 

Apr.  16,  all  alive.   May 
19,    5    budded    out. 

away. 

May    25,    all   appear 

dead.     May  31,  one 
with  fresh  buds  start- 

ing.      June     15,     all 

dead. 

J-  - 

Apr.   ii 

Root-shoot,  as  in  exp. 

50 

Ditto  

After    a    number    had 

148,  but  from  small 
plants. 

made   a  start    (Apr. 
25),  all  died  later. 

NOTE. — Exp.  i  to  17  were  done  jointly  with  Dr.  J.  E.  Kirkwood.    These  were  preliminary,  and  not 
under  critical  conditions. 


Vegetative  Reproduction. 


197 


TABLE  55. — Experiments  in  propagation  by  cuttings — Continued. 


Exp. 
No. 

Date. 

Parts  used. 

No. 
of  cut- 
tings. 

Conditions. 

Results. 

1  60 

IQ08 

May   19 

Twigs  20  cm.  long  .  .  . 

356 

Planted    reversed' 
in  a  "melga,"  *   2 

May  31,3  started:  June 
5,  46  started:  July  9, 

to  5  cm.  projecting 

55  started.     In  some 

above    surface     of 

cases  buds  started  10 

soil. 

cm.    below    surface. 

None  lived.    No  roots 

in  any  case. 

1610 

May    19 

Twigs    15    cm.  long, 

19 

Prepared  bed  of  lime- 

May 29.  5  started,  but 

leaves  not  removed. 

stone  soil.   Planted 

all    died    later.      No 

reversed. 

roots. 

i6ib 

May  19 

Ditto,       leaves      re- 
moved. 

35 

Ditto  

May  29,  4  started,  but 
all    died    later.      No 

roots. 

162 

May   19 

Twigs    15    cm.    long, 
foliage        trimmed 

20 

Ditto,  not  reversed.  .  . 

May    26-  June     5,      19 
started,      all      dying 

away. 

later.    No  roots. 

163 

May   19 

Roots,  2  to   10  mm. 

35 

Ditto  

June    10,   7  started,  of 

diameter. 

which   6   died.     One 

grew  well. 

1  On  the  theory  that  newer  tissues  might  be  able  to  regenerate  roots. 

2  A  melga  is  a  bed  with  a  deep  border  to  facilitate  irrigation  by  flooding.    Alfalfa  is  frequently 
irrigated  in  this  way. 

methods.  In  many  instances  they  will  produce  new  shoots,  the  size  of 
which  varies  directly  with  the  volume  of  the  piece.  Consequently,  exami- 
nation of  the  above-ground  parts  might 
easily  persuade  the  uninitiated  that 
growth,  including  that  of  the  roots,  had 
taken  place.  The  fact  remains  that  in 
no  case  had  the  pieces  regenerated  roots, 
and  in  consequence  the  cuttings  all  died 
sooner  or  later. 

3.  Root  -  cuttings    may   live    and 
become  permanently  established,  but 
under  the  conditions  used  the  number 
was  small  (plate  20,  Ai).    In  these,  too, 
new  shoots  may  be  produced  without  a 
commensurate  growth  of  new  roots,  and 
the  cuttings  may  therefore  die  after 
starting. 

4.  Sectorial  cuttings  made  by  split- 
ting the  lower  part  of  the  plant  in  such 
a  manner  as  to  involve  root  and  stem 
tissue  grow  most  readily  (plate  20,  fig. 
A,  2  to  4).    The  pieces  heal  completely 
without   decaying  (fig.  18),   and  new 
growths  of  normal  extent  under  irriga- 
tion will  be    formed,  these    flowering 
abundantly  the  first   season.      Under 
favorable  conditions  about  75  per  cent 
may  be  expected  to  live. 

5.  Stem-tissue  may  be  forced  to  regenerate  roots  by  planting  the 
basal  portions  of  plants,  trimmed  close  to  the  top  of  the  tap-root  (plate 
44,  fig.  A).     Branches  which  then  start  out  will  generally  behave  as  the 


5. — A  successful  sectorial  root-stem  cut- 
ting, showing  complete  healing. 


198  Guayule. 

basal  shoots  of  mariola  do  normally,  namely,  each  will  send  out  a  root 
from  near  its  base  (plate  44,  fig.  A),  which  becomes,  in  effect,  a  tap-root. 
Thus  these  branches  become  established  independently  of  the  parent  stock, 
and  may  be  separated  from  it  and  used  for  propagation.  This  occurs  in 
nature  very  exceptionally ,  but  more  readily  when  the  top  has  been  removed 
by  design  or  accident;  in  the  field,  however,  roots  are  normally  produced 
from  the  basal  portion  of  the  retono  chief  shoot  (fig.  n),  from  which  its 
root-system  proper  is  derived.  The  more  ready  production  of  roots  in  this 
manner  in  irrigated  plants  is  connected  with  the  larger  supply  of  water. 

It  is  seen  that  the  guayule  displays  a  marked  polarity  analogous  to 
that  found  in  plants  which  will  not  regenerate  roots  from  stems  when  ma- 
ture, but  will  do  so  when  young  (Cupressus,  vide  Goebel,  Organography, 
Engl.  ed.,  p.  45,  I),  and  to  certain  lilies  (Hyacinthus  sp.,  Goebel,  ibid.),  in 
which  bulbils  are  formed  from  the  lower  portion  of  a  severed  stem,  but 
not  above.  That  is  to  say,  the  expression  of  polarity  by  root-regeneration 
from  the  stem  is  definitely  restricted  to  a  particular  region  of  certain 
stems  only,  namely,  to  the  lowermost  zone  of  the  branches  of  the  second 
and  (probably)  higher  orders,  which  themselves  arise  from  a  narrow  zone 
of  the  chief  stem  just  above  the  tap-root. 

Shoot-regeneration  is,  by  contrast,  easy,  and  this  is  true  for  the  root, 
from  which  stem-primordia  are  absent.  It  does  not  appear  that  external 
stimulus  is  necessary,  for  wounding  the  cortex  of  the  root  in  situ  is  not 
followed,  in  any  of  my  experiments,  by  shoot-formation  at  the  point  of 
wounding.  Nevertheless,  as  in  many  plants,  a  complete  severance  of  a 
root  left  in  situ  is  frequently  followed  by  shoot-formation,  but  in  a  posi- 
tion determined  by  other  conditions,  such  as  dying  back,  resulting  from 
drought.  Thus  it  appears  that  the  notion  formulated  by  Miss  Kupfer 
(1907),  that  the  disposition  to  form  roots  is  much  more  generalized  than 
to  form  shoots,  does  not  include  cases  like  this  before  us,  which  need  eluci- 
dation as  much  as  any.  And  as  McCallum  (1905)  has  well  said,  the  prob- 
lem of  regeneration  is  more  especially  to  determine  the  cause  of  non- 
development  "of  parts"  in  the  normal  life  of  the  plant. 


CHAPTER  IX. 
THE  CULTIVATION  OF  GUAYULE 

Under  the  cultivation  of  guayule  must  be  included  all  operations 
intended  to  modify  the  relation  of  the  plant  to  its  environment.  These 
operations  may  be  forestal  or  cultural,  in  the  narrower  sense.  It  is  the 
purpose  of  this  chapter  to  set  forth  the  conclusions  as  to  the  possibilities 
which  have  presented  themselves  in  both  these  directions.  Although  only 
a  beginning  has  been  made  in  the  solution  of  the  many  difficult  practical 
problems  which  have  arisen,  the  more  immediately  insistent  questions 
involved  have  been  fairly  if  not  completely  answered.  The  difficulty  of 
practice  is  not  lessened  by  the  fact  that  the  problem  before  us  is  distinctly 
a  desert  one,  and  the  final  answer  to  many  questions  may  not  be  obtained 
for  many  years. 

FORESTAL  OPERATIONS. 
PRESENT  FIELD  OPERATIONS. 

Up  to  the  present  time,  with  only  very  few  experimental  exceptions, 
field  operations  have  been  confined  to  the  collection  of  shrub  in  the  great- 
est possible  amount  with  the  greatest  ease,  for  the  sake  of  the  immediate 
monetary  return.  This  has  had  both  a  bad  and,  in  less  degree,  a  good 
result.  In  many  places  where  shrub  had  been  taken  there  were  so  many 
small  plants  that  it  was  thought  that  it  would  not  pay  to  collect  them, 
and  these  will  serve  to  repopulate  the  areas  so  treated.  In  other  places, 
where  the  stand  consisted  only  of  large  plants,  nearly  every  vestige  has 
been  removed,  leaving  at  most  only  the  occasional  small  plants  to  lay  the 
foundations  for  the  future.  If  in  such  places  a  few  healthy  medium-sized 
plants  had  been  left  to  produce  seed,  as  common  sense  should  have  dic- 
tated, ground  that  will  be  barren  of  guayule  for  many  years  might  have 
been  repopulated,  at  any  rate  to  some  extent. 

The  method  which  has  ordinarily  been  used  is  to  pull  up  the  plant  by 
hand,  and,  while  the  method  of  cutting  it  off  at  the  surface  of  the  ground 
has  been  advocated  and  to  some  extent  practiced,  pulling  has  been  most 
largely  used.  But  in  very  rocky  areas,  where  the  plants  frequently  grow 
in  the  fissures  of  the  rock,  from  which  it  is  often  impossible  to  pull  them 
out,  the  peons  will  break  or  twist  off  the  top,  leaving  the  butt  in  the 
ground.  A  specific  case  of  this  kind  was  noted  by  me  in  a  part  of  the 
Sierra  de  Ramirez,  a  range  of  mountains  lying  partly  in  each  of  the  States 
of  Zacatecas  and  Durango,  opposite  Tanque  de  la  Pendencia.  On  first 
entering  the  guayule  area,  which  had  been  worked  in  the  winter  of  1907- 
08,  scarcely  any  guayule  was  to  be  seen,  but  further  search  discovered 
numerous  young  growths,  visible  with  difficulty  on  account  of  their  color 
when  seen  in  April  1909,  which  had  come  up  from  the  basal  portions  of 
plants  which  had  been  twisted  off.  Bare  as  this  ground  appeared  to  be 
of  guayule,  there  is  little  doubt  that  in  time  the  stand  will  be  replenished 
to  a  large  degree,  if  not  fully. 

199 


200  Guayule. 

In  the  Lomerio  de  Zorrillos,  some  leagues  further  south,  where  the 
substratum  is  made  up  of  fine  limestone  soil  containing  stones  of  various 
sizes,  it  is  easy  to  pull  the  plants  up,  and  here  all  the  larger  ones  were 
taken.  As  the  number  of  small  plants  was,  however,  very  great,  all  these 
were  left,  and  number  600  to  800  to  the  100  square  meters,  weighing  i  to  2 
tons  to  the  hectare.  This  condition  probably  represents  the  best  that  may 
occur  under  the  old  methods,  and  is  but  seldom  found.  In  many  spots 
from  which  larger  plants  had  been  taken,  pieces  of  root  left  by  breaking 
off  were  found  to  have  produced  retonos. 

The  work  of  pulling  up  the  guayule  is  done  by  peons  who  tie  the  shrub 
into  bundles,  make  up  burro-loads,  and  take  it  to  a  neighboring  "  campo  de 
guayule,"  a  field-center  of  operations,  where  the  shrub  is  baled  in  hand- 
presses.  From  here  it  is  hauled  in  wagons  to  the  most  accessible  shipping- 
point  on  the  railroad,  and  so  by  rail  to  the  factory. 

In  undertaking  to  harvest  the  shrub  from  a  particular  region,  the 
usual  method  is  to  let  a  contract  to  local  agents  who  can  command  the 
conditions,  which,  as  may  well  be  imagined,  are  often  severe  on  account 
of  the  great  distances  and  lack  of  water.  The  easiest  time  to  work  is 
while  the  ground  is  still  soft  from  the  rains  and  when  water  is  relatively 
plentiful,  and  it  happens  that  this  is  the  worst  possible  time  to  take  the 
plant  as  regards  its  rubber- content.  At  that  time  also  the  shrinkage  in 
weight  is  much  greater,  both  by  the  loss  of  a  greater  amount  of  water  in 
the  plant  and  the  larger  bulk  of  the  foliage. 

SUGGESTED  RULES  OF  PRACTICE. 

The  statement  will  not  need  defense  that  an  immediate  desideratum 
is  a  rationale  of  forestal  operation,  in  order  that  the  present  field  supply, 
already  much  reduced  from  the  original  stand,  may  be  kept  from  being 
well-nigh  wiped  out.  The  data  upon  which  rules  of  procedure. must  be 
based,  in  the  absence  of  still  necessary  extensive  quantitative  study  of 
treated  areas,  have  been  presented  in  Chapter  IV.  The  general  practice 
indicated  by  the  experiments  recorded  will  therefore  be  stated  here. 

1 .  Guayule  should  be  gathered  by  cutting  it  off  at  the  level  of  the 
ground.    That  which  is  allowed  to  project  above  the  surface  will  die  back 
more  or  less  and  be  an  economic  loss,  as  these  parts  represent  a  substan- 
tial proportion  of  the  weight  of  the  plants.    The  cutting  should  be  done 
with  a  sharp  grubbing-hoe  (talacho) ,  a  method  which  is  easier  on  the  men, 
as  well  as  contributing  to  the  preservation  of  the  stand  of  plants.    It  is 
practically  certain  that  new  shoots  will  arise  from  many  of  the  parts  left 
in  the  ground,  and  these,  during  the  first  season,  will  produce  flowers,  the 
seeds  from  which  will  help  to  repopulate  the  area. 

It  has  recently  been  suggested  by  Escobar  (1910)  that,  after  cutting, 
a  shallow  depression  be  cut  in  the  soil  about  the  remaining  root,  for  the 
purpose  of  catching  the  run-off,  thus  increasing  the  water-supply.  Further 
operations  (terracing  or  furrowing  along  the  contour  lines) ,  designed  to 
hold  back  the  run-off,  are  also  recommended.  In  many  situations  it 
would  be  difficult  to  carry  out  schemes  of  this  kind. 

2.  Only  plants  40  cm.  or  more  in  height  should  be  taken  on  the  first 
cutting.    Five  years  later  there  should  normally  be  a  crop  of  40  cm.  plants, 


The  Cultivation  of  Guayule.  201 

which  may  then  be  taken.  Between  30  and  40  cm.  the  maximum  econo- 
mic efficiency  of  growth  obtains,  and  this  lies  between  10  and  15  years  of 
age.  Fifteen  years  is  therefore  the  rotation  period,  but  as  the  growth  effi- 
ciency of  a  plant  falls  after  this  age  has  been  reached,  these  plants  must  be 
removed  each  fifth  year.  The  advantage  of  this  rule  is  to  be  expected  not 
only  in  the  growth  of  the  plants  already  there,  but  also  in  the  great  effi- 
ciency of  seeding.  The  question  has  been  raised  as  to  the  possible  increase 
of  efficiency  of  germination  by  partial  or  total  clearing  of  the  land,  thus 
removing  the  factor  of  competition. 

3.  Removal  of  the  vegetation  other  than  guayule.  It  is  too  early  to 
make  any  definite  statements  as  to  the  value,  even  with  regard  to  the 
well-being  of  the  mature  plant,  of  clearing  operations  on  guayule  lands. 
The  experiments  which  have  been  initiated  involve  an  area  of  about  75 
acres,  which  were  well  cleared  of  all  vegetation  excepting  the  guayule,  the 
"palms"  (palma  samandoca)  which  produce  fiber,  and  the  few  cacti,  of 
large  species,  which  occupy  little  area  and  do  not  constitute  an  aggressive 
element  in  the  vegetation.  The  clearing  of  the  land  has  the  effect  of  loos- 
ening the  more  superficial  layers  of  soil  generally,  and  to  some  depth  in 
spots.  On  general  grounds  this  ought  (i)  to  remove  competition  with 
other  plants,  which,  as  has  been  shown  elsewhere,  is  not  insignificant  and 
frequently  constitutes  a  real  menace  to  the  guayule,  necessitating  partial 
clearing,  at  least.  This  competition  of  course  relates  especially  to  the 
water-content  of  the  soil.  Unless  the  removal  of  the  covering  allows 
greater  washing  than  in  any  event  occurs,  it  should  render  more  water 
available  for  the  guayule  and  thus  enhance  growth.  It  must  not  be  for- 
gotten, however,  that  a  much  greater  growth  is  correlated  with  reduced 
activity  in  secretion  of  rubber,  either  directly  or  by  reducing  the  volume 
of  the  rubber-bearing  tissues,  as  has  been  brought  out  in  the  discussion 
of  plants  under  irrigation  and  from  areas  of  greater  rainfall  (Chapter  V) . 
(2)  It  is  important  also  to  know  what  effect  the  removal  of  the  vegetation 
has  upon  the  crop  of  seedlings.  The  evidence  so  far  obtained  appears  to 
favor  the  clearing  of  the  land.  I  refer  especially  to  the  census  of  seedlings 
made  at  Station  2  (page  70),  in  which  are  recorded  numbers  of  seedlings 
far  in  advance  of  those  found  elsewhere.  As  to  the  question  of  protec- 
tion afforded  young  seedlings  by  the  shade  of  other  plants,  of  no  small 
importance  in  many  cases,  as  has  been  repeatedly  indicated  by  studies  at 
the  Desert  Botanical  Laboratory,  it  may  be  concluded  that  the  number 
of  seedlings  which  survive  a  six  months'  drought,  as  observed  by  myself 
in  April  1909,  is  sufficient  to  warrant  the  statement  that  not  enough  suc- 
cumb to  unfavorable  conditions  to  neutralize  the  good  effects,  as  seen  in 
the  surviving  numbers.  It  seems  probable,  therefore,  that  clearing  the 
land  of  other  vegetation,  saving  the  species  above  mentioned,  is,  on  the 
whole,  beneficial  to  the  guayule. 

As  to  the  specific  question  of  the  response  in  growth,  all  that  can  be 
said  at  this  time  is  guesswork.  The  areas  which  were  cleared,  as  it  hap- 
pened, were  subject  to  severe  droughts  from  the  time  of  clearing  in  the 
winter  of  1 907-08  till  the  summer  of  1 909.  It  is  hoped  that  the  abundant 
rainfall  of  the  season  now  drawing  to  a  close  will  enable  us  to  form  some 
conclusion  on  this  point. 


202  Guayule. 

HARVESTING  PERIOD. 

The  question  of  the  variation  in  the  relative  rubber-content  of  the 
guayule  according  to  the  time  of  the  year  is  undoubtedly  of  more  impor- 
tance than  is  at  present  appreciated.  The  loss  arising  from  this  cause, 
moreover,  can  not  be  detected  by  the  chemical  control  of  a  factory  labora- 
tory, for  the  reason  that  the  new  succulent  growths  when  dried  add  but 
little  to  the  weight  of  the  plant,  while  their  capacity  for  rubber-secretion 
is  indicated  by  their  living  volume.  The  "shrinkage"  between  field  and 
factory  referred  to  by  the  manufacturer  is  equally  inefficient  as  an  indi- 
cator of  the  loss,  in  a  practical  sense ;  shrinkage  consists  of  all  kinds  of  loss 
in  handling  and  transportation  from  the  field  to  the  factory,  an  important 
economic  factor  which,  while  including  the  loss  under  consideration,  leaves 
it  undiscoverable. 

An  element  of  uncertainty  arises  from  the  different  moisture-content 
of  the  shrub  at  various  seasons.  Thus,  the  shrinkage  in  weight  from  dry- 
ing in  field  plants  is  from  20  to  25  per  cent  (exactly  in  my  determinations 
between  22  and  23  per  cent)  during  drought;  in  irrigated  plants  it  is  as 
high  as  50  per  cent.  In  August  1908,  at  the  height  of  the  growing-season, 
the  water-content  ranged  between  25  and  50  per  cent,  averaging  in  the 
neighborhood  of  35  to  40  per  cent,  as  high,  nearly,  as  in  irrigated  plants, 
in  which  it  rarely  falls  below  40  per  cent,  and  is  usually  about  50  per  cent. 
In  addition  to  this,  the  weight  of  the  additional  leaves  in  summer  is  not 
negligible.  I  shall  therefore  venture  to  state  with  some  insistence  that, 
assuming  normal  distribution  of  rainfall,  the  gathering  of  shrub  during 
summer  months  and  for  several  months  thereafter  can  mean,  practically, 
only  the  total  loss  of  the  rubber  accretion  of  a  whole  year.  The  small 
amounts  of  rubber  undoubtedly  present  in  the  newer  growths  can  scarcely 
be  recovered  by  mechanical  means,  while  the  ready  breakage  of  the  slender 
and  weak  twigs  of  recent  growth  would  in  any  event  result  in  a  loss. 

Another  consideration  is  involved  also.  The  germination  during  the 
growing-season  results  in  the  annual  crop  of  young  seedlings,  the  greater 
part  of  which,  on  account  of  the  numbers  and  small  size,  would  undoubt- 
edly be  destroyed  by  the  peons  at  work  collecting  shrub.  Aside  from  this, 
the  peons  should  be  not  only  instructed  but  compelled  to  work  carefully, 
so  as  not  to  destroy  the  small  plants. 

RESEEDING  BARREN  GROUND. 

Land  from  which  guayule  has  been  completely  removed  may,  under 
favorable  conditions,  be  restocked  by  the  simple  operation  of  reseeding. 
Whether  the  cost  of  doing  this  would  be  justified,  however,  is  doubtful, 
since  an  area  of  any  size  would  require  an  immense  amount  of  seed,  which 
at  present  it  is  difficult  to  obtain  in  quantities,  and  since  the  percentage 
of  germination  under  natural  conditions  would  be  very  small. 

Whether  the  business  view  will  see  a  sufficient  monetary  recompense 
in  the  returns  from  following  the  procedure  above  recommended  is  not 
the  present  problem.  Local  conditions  vary  too  much  to  solve  it  in  general 
terms.  This  much,  however,  may  be  said:  that  the  rules  of  operation  out- 
lined are  dependable  in  the  degree  indicated,  and  that  the  disregard  of 
them,  or  of  some  equally  or  more  efficient  ones,  will  only  lead  to  the  prac- 
tical extermination  of  the  plant. 


The  Cultivation  of  Guayule.  203 

CULTURAL  OPERATIONS. 

Although  it  will  be  granted  that  forestal  operations  are  of  immediate 
and  great  importance  for  the  preservation  of  the  natural  stand  as  a  source 
of  revenue  for  as  long  a  period  as  possible,  the  ultimate  and  adequate 
solution  of  the  production  of  guayule  shrub  lies  in  its  successful  cultiva- 
tion. That  this  is  possible  seems  at  the  present  not  to  be  overstating  the 
case.  The  abundant  and  ready  growth  of  guayule  under  irrigation,  its 
drought-resistant  qualities,  its  consequent  adaptability  to  comparatively 
meager  irrigation,  if  this  condition  is  imposed,  and  its  ability  to  secrete 
rubber,  though  in  relatively  smaller  quantities  per  unit- volume  of  tissue, 
under  irrigation  properly  alternated  with  drought,  are  established  facts. 
It  remains,  therefore,  to  test,  on  a  larger  scale  than  has  hitherto  been 
attempted,  what  may  be  done  to  establish  the  culture  of  the  plant  on  an 
economic  basis.  But  in  stating  the  positive  basis  for  success  the  difficulties 
must  not  be  underestimated.  These  will  be  indicated  in  what  follows, 
and  it  will  suffice  here  to  point  out,  in  a  word,  that  the  great  difficulty  lies 
in  the  initial  work  of  establishing  the  plants,  which  necessitates  water.  It 
would  be  useless  to  attempt  cultural  operations  without  it. 

SEED. 

Should  it  turn  out  finally  that  the  raising  and  the  transplanting  of 
seedlings  is  a  desirable  method  of  procedure,  the  obtaining  of  a  sufficient 
amount  of  seed  will  be  an  important  desideratum.  At  present  it  would 
be  necessary  to  collect  seed  from  the  field.  This  is  costly  and  uncertain. 
Experience  has  shown  that  the  ripening  of  seed  in  the  field  is  uneven ;  much 
of  it  quickly  falls  off,  and  the  most  satisfactory  places  to  collect  are  fre- 
quently far  removed  from  habitations.  Hand-picking  is  slow,  but  could 
be  rendered  more  efficient  by  the  arrangement  of  a  device  of  wire  and 
cloth,  in  two  pieces,  to  be  held  under  the  shrub,  from  which  the  seed  could 
then  be  dislodged  by  light  beating.  It  seems,  however,  unlikely  that  any 
field  method  of  collecting  seed  will  be  as  satisfactory  as  its  production  by 
irrigated  plants,  which,  in  the  climate  of  North  Zacatecas,  will  ripen  seed 
for  the  greater  part  of  the  year.  The  ripening  of  a  large  amount  at  one 
time  will  render  rapid  collection  easier.  Some  such  device  as  suggested 
will  in  any  event  be  necessary,  as  the  seed  must  be  collected  and  submitted 
to  optimum  conditions  in  order  to  get  the  maximum  germination.  It  has 
been  found  that  the  ordinary  conditions  of  growth,  even  under  irrigation, 
are  not  efficient  for  this  result.  The  advantage  of  growing  plants  under 
irrigation  is  in  the  convenience  and  economy  in  obtaining  seed,  and  not 
in  its  superior  quality.  Kirkwood  (ipioa)  found  that  the  number  of  good 
seed  from  irrigated  plants  does  not  exceed  17  per  cent,  and  this  is  some- 
times, but  not  often,  surpassed  in  field  plants. 

THE  RAISING  OF  SEEDLINGS. 

The  small  size  of  the  seedlings  and  their  tender  character  when  young 
make  it  necessary  to  handle  them  with  considerable  care.  The  precise 
conditions  for  their  successful  culture  have  been  studied  by  Dr.  Kirkwood 
(igioa)  and  by  myself,  and  from  these  experiences,  but  more  particularly 
from  my  own,  the  following  practical  suggestions  are  made: 


204 


Guayule. 


The  probable  necessity  of  transplanting  large  numbers  of  seedlings 
at  a  very  great  risk  of  loss  led  me  to  adopt  experimentally  a  scheme  used 
in  the  tropics,  where  the  hollow  joints  of  bamboo  are  used  as  flowerpots 
in  which  to  raise  cacao,  coffee,  and  other  seedlings.  When  ready  for  plant- 
ing in  the  grove  the  whole  pot  is  planted,  and  the  decay  of  this,  aided  by 
fracture,  sets  the  roots  free  without  any  disturbance.  In  a  preliminary 
way  the  joints  of  "carrizos"  (Arundo  donax)  were  tried,  but  proved  too 
small.  Combining  this  idea  with  that  of  the  paper  flowerpot,  a  unit  sys- 
tem of  wooden  trays  and  paper  tubes  was  devised,1  the  tubes  being  i 
square  inch  in  transverse  section  and  6  inches  long  (plate  45,  fig.  A).  As 
trials  with  these  taught  that  they  afforded  too  little  room  for  the  horizon- 
tal development  of  roots,  a  comparative  test  with  similar  tubes  of  4  square 
inches  transverse  section  was  carried  out  under  identical  conditions.  A 
tray  20  by  28  inches  inside  measure  and  6  inches  deep  was  filled  with 
these  tubes  (plate  45,  fig.  B),  the  whole  being  filled  with  unsifted  limestone 
soil  in  which  there  were  a  great  many  small  fragments  of  caliche  and 
stones.  The  tray  was  placed  in  a  melga  and  watered  by  subirrigation.  The 
surface  was  shaded  at  a  height  of  4  cm.  by  a  thin  cotton  cloth  supported 
in  a  frame.  The  shade  was  raised  or  lowered  as  the  surface  appeared  to 
need  more  or  less  air,  so  as  to  check  the  growth  of  fungi  (a  Coprinus  sp. 
was  very  frequent  in  the  decaying  paper  of  the  tubes) ,  among  which  one 
species,  at  least,  caused  damping-off.  On  February  16,  1908,  1.5  ounces 
of  seed  were  sown.  The  germinations  were  as  shown  in  table  56. 

TABLE  56. 


Date. 

Germina- 
tions for 
period. 

Total 
germina- 
tions. 

Loss. 

Net  total 
living 
seedlings. 

Feb.  26  

11:::: 

29  
30  
Mar.     8 

2 
2 

18 
5° 
32 
300 

2 

4 

22 

72 
104 

404 

2 

4 

22 
72 

12  

16  

IOO 
21 

504 
525 

41 

484 

The  appearance  of  these  seedlings  is  shown  in  plate  45,  figs.  C,  D,  from 
which  it  will  be  seen  that  a  good,  fairly  even  stand  of  sturdy  seedlings 
(plate  46,  fig.  A)  was  obtained.  The  size  of  the  tubes  used  was,  of  course, 
a  compromise,  but  fig.  19,  A,  shows  that  a  sufficiently  satisfactory  root- 
system  can  grow  in  them,  though  of  course  by  no  means  as  good  as  when 
the  roots  have  normal  freedom  (fig.  19,  B),  which  in  any  case  is  neither 
desirable  for  practical  purposes  nor  expected.  The  tray  held  140  tubes, 
from  which  it  is  seen  that  there  was  an  average  of  about  3  seedlings  to  the 
tube.  The  unevenness  was  due  to  the  removal  of  seed  from  its  original 
position  by  rain  or  occasional  surface-watering,  which  is  desirable  to  aid 
in  preventing  too  rapid  caking  of  the  surface.  To  prevent  this  movement 
of  seed  the  surface  should  be  as  level  as  possible.  The  margin  of  the  tray 


By  Capt.  L.  C.  Andrews. 


,        F&ES32&SS  ! 

lisfe^^ 


<  u 


The  Cultivation  of  Guayule. 


205 


should  preferably  be  somewhat  higher  than  the  surface  of  the  soil,  as  this, 
in  addition  to  enabling  one  to  manage  the  shade  better,  prevents  the  dry- 
ing out  of  the  soil  near  the  edge,  in  consequence  of  which  the  germination 
is  not  so  good. 

The  subirrigation  may  be  managed  best  by  placing  the  trays  in  melgas 
of  a  depth  sufficient  to  bring  the  surface  of  the  water  to  the  level  of  the  top 
of  the  soil  in  the  tray.  In  order  that  the  water  may  gain  free  access  to  the 
soil  the  sides  of  the  trays  must  be  provided  with  a  number  of  holes. 


FIG.  19. — A,  the  roots'of  two  seedlings  grown  in  4-square-inch  paper  tubes;  B.  a  root-system  of  about 
the  same  age,  growing  in  a  box.  X9/ao. 

Despite  the  apparent  indifference  of  guayule  seed  to  the  temperatures 
recorded  by  Kirkwood  (19100) ,  seeds  germinate  more  promptly  and,  what 
is  more  important,  the  seedlings  make  a  much  more  rapid  growth  during 
the  summer  months,  as  my  experiments  in  July  and  August  1908  showed. 
In  winter,  also,  the  seedlings  were  frequently  killed  in  part  by  frost,  in  part 
by  a  storm  of  hail,  and  were  more  subject  to  damping-off.  The  heavy 
rains  of  summer  also  prove  more  or  less  destructive,  and  it  was  found  that 
the  seedlings  with  the  shortest  hypocotyls  survived  the  best.  For  this 
reason  as  thin  a  shade  as  possible  is  desirable,  the  object  of  this  being  to 
preserve  the  superficial  soil-moisture  and  to  cut  down  the  light  as  little  as 
possible. 


206 


Guayule. 


When  it  is  desired  to  transplant  the  seedlings,  the  tubes  will  be  found 
to  be  soft  and  partially  decayed,  so  that  they  may  be  torn  by  slight  pres- 
sure when  being  placed  in  the  ground.  This  will  favor  a  prompter  adjust- 
ment to  the  new  conditions.  The  loss  will  vary  and  can  not  be  foretold, 


FIG.  20. — The  same  root-system  as  shown  in  fig.  19,  B,  projected  on  a 
horizontal  plane. 

but  with  care  should  be  small.  As  time  did  not  permit  an  extended  trial 
of  this,  however,  I  am  unable  to  state  economically  valuable  results, 
though  some  indication  has  been  had  from  the  following: 

Experiment  79. — Of  449  seedlings  (small  plants  i  to  5  years  old), 
transplanted  into  irrigated  ground  by  a  peon,  300  lived.  The 
transplanting  was  done  Nov.  26,  1907,  care  being  taken  to  pre- 
vent the  roots  from  drying  out,  and  the  ground  was  well  irri- 
gated. On  Feb.  16,  1908,  the  first  indications  of  growth  were 
seen,  but  the  plants  started  unevenly,  some  not  showing  signs 
of  new  growth  until  Apr.  9. 

Experiment  159. — May  3,  1908,  5  seedlings  were  transplanted  in  i- 
inch  tubes,  the  upper  5  cm.  of  the  tube  only  being  preserved. 
All  lived  and  grew  well  till  last  observed,  Sept.  1908. 

Experiment  153. — Apr.  8,  1908.  Of  14  small  seedlings  (epicotyl  10 
mm.  long  in  the  largest)  transplanted  into  a  prepared  bed,  12 
grew  well,  2  died. 

Experiment  164  (J.  E.  Kirkwood). — On  May  13  a  bed  was  prepared 
by  digging  up  the  soil  and  flooding  to  a  depth  of  4  inches.  On 
the  following  day  i -square-inch  paper  tubes  containing  seedlings 
an  inch  high  were  set  in  the  wet  ground  their  full  length.  These 
plants  had  been  grown  in  the  tubes  from  seed  and  were  some 
two  months  old.  64  of  these  were  planted,  and  nearly  all  lived. 

Experiment  165  (J.  E.  Kirkwood). — 50  tubes  containing  plants  of 
the  same  lot  as  the  preceding  were  set  in  relatively  dry  soil 
which  showed  visible  moisture  an  inch  or  so  below  the  surface. 
This  was  done  May  1 4  and  the  ground  received  water  to  saturate 
several  inches  on  the  i8th  and  igih.  A  few  of  these  survived. 


The  Cultivation  of  Guayule,  207 

Experiment  166  (J.  E.  Kirkwood).— Bed  prepared  as  above  and  cov- 
ered with  4  inches  of  water.  On  the  following  day  250  plants 
(seedlings)  of  three  months  or  less  were  transplanted  into  this 
bed.  These  plants  received  no  more  water  than  what  was  given 
at  the  start,  in  order  to  test  this  practice  in  the  transplanting. 
In  five  days  these  plants  appeared  to  be  dead. 

Experiment  167  (J.  E.  Kirkwood). — Bed  prepared  as  before  and 
watered  to  saturation.  Into  this  15  young  plants  were  set  on 
May  15  and  immediately  watered  by  flooding.  The  bed  was 
watered  again  on  May  16.  Nearly  all  of  these  lived.  It  resulted 
that  in  164  and  167,  in  which  abundant  watering  was  had  at  the 
start,  nearly  all  of  the  plants  lived.  In  the  others  nearly  all 
failed. 

Transplanting  cultivated  seedlings  into  the  natural  habitat  was  tried, 
but  the  plants  were  destroyed  by  goats.  The  operation  is  fraught  with 
much  difficulty  on  account  of  the  character  of  the  ground,  and  would  not 
justify  itself  practically. 

It  may  properly  be  said  that  the  raising  of  guayule  seedlings,  more 
particularly  during  the  first  few  weeks,  is  not  a  mere  rule-of-thumb  pro- 
cedure. One  has  to  watch  them  with  care  and  learn  their  idiosyncrasies. 
Later  they  become  quite  resistant  and  may  be  handled  much  more  easily. 

The  best  soil  for  them,  so  far  as  experiments  by  Kirkwood  (i  9100)  and 
myself  have  shown,  is  the  limestone  soil  of  their  natural  habitat  (plate 
1.6).  Soil  which  contains  a  good  deal  of  humus  appears  unfavorable  for 
young  seedlings,  as,  among  other  difficulties,  damping-off  is  very  preva- 
lent. However,  they  were  found  to  have  germinated  abundantly  after 
lying  in  such  soil  for  seven  months,  and  grew  well,  though  it  must  not  be 
forgotten  that  the  soil  must  have  suffered  considerable  change  by  leaching 
and  chemical  action  in  the  interval.  The  action  of  fertilizers  has  not  been 
tried  as  yet  either  on  seedlings  or  mature  plants.  Recent  experiments 
have,  however,  shown  that  guayule  will  grow  well  in  a  noncalcareous  soil, 
and  respond  readily  to  sodium  nitrate. 

The  presence  of  small  stones  in  the  soil  appears  on  the  whole  to  be  an 
advantage.  The  following  experiments  were  done  to  test  this  point: 

Experiment  138,  Jan.  24,  1908. — Into  three  root-cages  with  sloping 
glass  sides  three  lots  of  seedlings  of  about  equal  size  were  trans- 
planted. One  (I)  of  these  contained  very  finely  sifted  limestone 
soil;  the  second  (II),  similar  soil  mixed  half  and  half  with  fine 
angular  gravel  of  limestone;  the  third  (III)  was  filled  with  the 
same  fine  soil  and  coarse  gravel  (i  cm.  ave.  diam.),  the  gravel 
occupying  the  space  that  it  would  without  the  soil.  Feb.  22, 
length  of  tap-root  in  (I)  50  mm.;  (II)  50  to  60  mm. ;  (III)  60  to 
120  mm.  These  measurements  represent  differences  in  rate  of 
growth  of  the  tap-root,  which  was  about  the  same  length  in  all 
at  the  beginning  of  the  experiment. 

April  5,  the  individual  measurements  of  the  roots  were  as  follows: 
(I)   10,  10,  10,  10,  12.5,  14,  17  cm.    Average,  11.9  cm. 
(II)   20,  17,  1 6,  13,  14,  15  cm.    Average,  15.8  cm. 
(Ill)  30,  31,  26,  23,  26,  17,  14,  14,  20,  25  cm.    Average,  22.6  cm. 

These  differences  were  reflected  in  the  aerial  development,  those  of 
III  being  obviously  in  advance  of  the  others. 


208  ;  [Guayule. 

Experiment  1390. — Two  seedlings  of  nearly  equal  size  were  planted 
January  24,  1908,  in  a  5 -gallon  oil-can,  half  of  which  contained 
a  soil  made  up  of  coarse  gravel  and  fine  soil  (of  the  latter  only 
so  much  as  would  go  between  the  gravel) ,  while  the  other  half 
contained  uniform,  finely-sifted  soil  of  the  same  kind.  The 
watering  was  equal  for  both  sides,  and  sufficient  to  keep  an 
abundance  of  water  available.  The  subsequent  growth  in  the 
plant  in  gravelly  soil  was  very  much  more  marked,  as  shown  in 
the  left-hand  plant  on  plates  18  to  20,  the  limit  of  growth  for 
the  year  being  nearly  reached  in  four  months.  This  plant,  which 
weighed  8  ounces,  produced  fully  2,000  seeds.  The  development 
of  roots  was  correspondingly  greater  in  the  gravelly  soil,  and 
careful  removal  of  the  roots  showed  that  they  were  confined 
chiefly  to  this  soil,  though  occasional  roots  of  each  plant  reached 
over  into  the  territory  of  the  other.  However,  it  should  be  noted 
that  there  appeared  to  be  a  tendency  of  the  roots  in  gravelly  soil 
to  grow  toward  the  fine  soil,  as  seen  in  plate  20,  fig.  A,  in  which 
the  plants  are  oriented  with  respect  to  each  other  as  they  grew. 
In  these  experiments,  therefore,  the  gravelly  soil  was  more  favor- 
able to  root-development,  a  result  which  appears  to  harmonize 
with  agricultural  practice. 


IRRIGATION. 

If  large  numbers  of  seedlings  are  to  be  raised,  the  method  of  watering 
will  introduce  a  material  element  of  expense,  aside  from  the  cost  of  the 
water.  Hand-watering  of  the  surface  would  prove  to  be  laborious  and 
expensive.  For  this  reason  a  method  of  subirrigation  was  tried,  with  the 
results  as  stated  above.  Additional  evidence  is  as  follows: 

Experiment  141. — To  test  the  relative  value  of  subirrigation,  with 
and  without  shade,  as  compared  with  surface  watering.  Four 
trays  with  i-inch  paper  tubes  (plate  45,  fig.  A)  were  filled  with 
limestone  soil  mixed  with  gravel,  each  sown  with  i  ounce  of  seed. 
(I)  Placed  on  the  surface  of  the  ground  and  shaded  by  a  thin 

white  muslin  screen. 
(II)  The  same,  but  without  shade. 

(III)  Placed  in  a  melga,  and  shaded  as  above. 

(IV)  The  same  as  III,  without  shade. 

Ill  and  IV  were  watered  by  subirrigation ;  I  and  II  by  surface  water- 
ing, and  served  as  a  check  on  III  and  IV.  It  was  noted  that  it 
was  very  difficult  to  keep  II  wet  enough.  The  surface  of  IV 
was  never  dry. 

In  both  shaded  trays  the  germination  was  far  in  excess  of  that  in  the 
control.  In  both  the  subirrigated  trays  taken  together,  the  germination 
was  over  twice  that  in  the  surface-watered  trays,  though  it  was  slightly 
more  in  the  shaded,  surface-watered  tray  than  in  the  unshaded,  subirri- 
gated tray.  The  result  indicates  clearly  that  subirrigation  with  shade  is 
the  most  favorable  of  the  four  conditions.  It  should  be  noted  that  tray  III 
was  left  unshaded  after  February  13,  in  order  to  avoid  extreme  etiolation, 
and  this  may  have  lowered  the  subsequent  rate  of  germination  without 
vitiating  the  general  result. 


The  Cultivation  of  Guayule. 


209 


TABLE  57. 


> 

lumbers  of 

seedlings  in- 

Tray  I. 

Tray  II. 

Tray  III. 

Tray  IV. 

Feb.    7  . 

6 

18 

8      

g 

g 

10   

21 

12 

58 

3 
25 

II   

28 

1  1 

12 

16 

J7 

6 

11 

14   

8 

7 

I38 

3 

17    

15 

2 

15 

4 

19    

3 

0 

5 

15 

22    

6 

2 

12 

9 

24    

5 

0 

0 

12 

Mar.  16  

21 

19 

8 

51 

Totals  
Less  loss  to  Mar.  16  .  . 

170 
13 

11 

283 

35 

I84 
40 

Total  alive  .    . 

I  E7 

248 

TRANSPLANTING. 

Another  method  of  getting  a  stand  of  guayule  started  and  having  the 
advantage  of  speed  is  by  transplanting  field  plants  into  irrigated  ground. 
Experience  has  taught  that  it  is  of  little  use  to  attempt  to  preserve  the 
aerial  part  of  plants  of  any  size,  and  that  even  small  ones  frequently  die 
back.  Of  a  plantation  of  some  hundreds  of  individuals  so  treated  (at 
Caopas),  scarcely  25  per  cent  grew,  but  upon  cutting  them  back  a  consid- 
erable additional  number  revived  (plate  46,  fig.  B).  If  it  should  be  found 
desirable  for  any  reason  to  start  a  crop  of  guayule  from  field  plants,  the 
best  method  is  to  cut  back  to  the  top  of  the  tap-root  and  send  the  tops  to 
the  factory  for  extraction.  The  returns  from  these  would  go  far  toward 
the  expense  of  the  operations.  It  is  difficult  in  any  event  to  start  stocks 
unless  previously  pollarded. 

The  portions  to  be  planted  should  be  handled  as  rapidly  as  possible, 
being  kept  from  drying  out  by  means  of  wet  burlaps,  or  some  such  means. 
They  should  be  planted  deeply,  the  cut  surface  being  no  higher  than  the 
surface  of  the  soil,  and  they  should  then  be  thoroughly  irrigated.  The 
question  as  to  the  amount  of  water  which  may  be  used  without  doing  them 
damage  is  answered  by  the  simple  experiment  (exp.  145,  Feb.  9,  1908)  of 
putting  a  number  of  plants  into  water  with  their  roots  and  basal  part  of 
the  stem  totally  submersed.  In  four  days  numerous  actively  growing  len- 
ticels  were  to  be  seen  on  the  submersed  stem,  and  on  March  14  a  rootlet 
10  mm.  long  had  grown  from  one  plant,  while  others  had  started.  By 
February  24  rootlets  6  to  8  mm.  long  occurred  on  the  upper  parts  of  the 
tap-root,  and  even  roots  of  the  third  order  were  subsequently  formed. 
There  was  no  sign  of  disorganization,  so  that,  unless  the  soil  itself  should 
introduce  unfavorable  elements,  we  may  believe,  as  indeed  experience  in 
general  shows,  that  the  guayule  can  stand  abundant  water. 
14 


210  Guayule. 

The  best  time  of  the  year  for  transplanting,  as  shown  by  the  prompter 
responses  of  the  experiments  cited  in  Chapter  VI,  is  in  late  spring  and 
in  summer,  when  the  warmer  night -temperatures  aid  in  stimulation.  The 
differences  in  this  regard  were  very  noticeable  and  showed  conclusively 
that  winter,  in  North  Zacatecas  at  any  rate,  is  unfavorable  for  cultural 
operations  of  any  kind. 

The  advantage  of  cutting  back  to  the  region  of  the  tap-root,  in  addi- 
tion to  avoiding  the  loss  from  dying  back,  is  to  be  had  in  the  behavior 
which  I  have  described  at  some  length  in  Chapter  VI,  namely,  the  produc- 
tion of  basal  shoots  which  root  independently.  These  shoots  will  be  pro- 
duced the  more  frequently  the  nearer  the  tap-root  the  cut  is  made.  As 
also  the  guayule  frequently  sends  out  new  shoots  before  any  new  roots 
have  been  formed,  there  is  less  likelihood  that  these  will  exhaust  the  avail- 
able moisture  when  the  whole  of  the  transplanted  portion  is  covered  with 
soil. 

HARVESTING  CULTIVATED  GUAYULE. 

It  is  almost  gratuitous  to  say  anything  about  this  topic,  as  up  to  the 
present  time  the  facts  have  not  warranted  cultural  trials  on  a  scale  suffi- 
cient to  make  available  a  crop  of  anything  but  limited  experimental  size. 
We  are  justified,  however,  in  drawing  a  few  conclusions  from  the  facts 
which  have  been  brought  to  light  in  the  present  paper. 

Assuming  that  the  amount  of  rubber  ultimately  produced  by  guayule 
under  irrigation  is  sufficient  to  warrant  its  culture,  it  seems  clear  that  the 
methods  of  harvesting  should  be  approximately  as  follows:  The  new 
growths,  say  of  two  years,  of  plants  about  a  meter  in  spread,1  may  with 
advantage  be  removed  by  a  cutting  instrument,  so  as  to  leave  the  butt 
undisturbed  to  shoot  out  afresh.  The  branches  which  have  rooted  can 
then  be  removed  by  hand  simply  by  breaking  them  away,  and  replanted. 
These  are  usually  supplied  with  a  strong  root  which  can  be  pulled  up  with- 
out severe  damage.  In  this  way  the  cultivated  stand  may  be  increased 
ad  libitum,  provided  areas  with  sufficient  water  are  at  hand. 

CATCH  CROPS. 

Immense  areas  of  land  are  available  in  the  Mesa  Central  of  Mexico, 
and  doubtless  elsewhere,  where  "riego  temporal"  is  practiced.  This  sys- 
tem of  irrigation  consists  of  ditches  to  catch  the  run-off,  leading  it  to  the 
fields.  The  behavior  of  guayule  would  seem  to  justify  the  belief  that  this 
plant  could  be  grown  for  a  sufficient  period,  say  two  or  three  years,  in  such 
irrigable  areas,  and  the  expense,  in  part  at  any  rate,  offset  by  growing  corn 
or  some  other  suitable  plant,  as  a  catch  crop.  The  guayule,  when  of  suffi- 
cient size,  should  then  be  "laid  by"  to  endure  a  period  of  drought  till  it 
becomes  usable,  when  it  could  be  cut  as  suggested,  and  restarted.  This 
suggestion,  and  it  is  that  and  no  more,  deserves  a  serious  trial. 

1  Assuming  the  conditions  which  have  constantly  been  referred  to  in  this  work. 


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NOTE.— A  few  of  the  above  citations  have  been  introduced   during  proof  reading  for  the  sake  of 
completeness. 


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